Specify necessary functions and expected behavior of interface between the loader library and ICDs and layers for Windows, Linux and Android based systems. Also describe the application visible behaviors of the loader.
Application, Vulkan driver and Vulkan layer developers.
Any developers interested in understanding more about loader and layer behavior and architecture.
Support multiple ICDs (Installable Client Drivers) to co-exist on a system without interfering with each other.
Support optional modules (layers) that can be enabled by an application, developer or the system and have no impact when not enabled.
Negligible performance cost for an application calling through the loader to an ICD entry point.
Vulkan is a layered architecture. Layers can hook (intercept) Vulkan commands to achieve various functionality that a Vulkan driver (aka ICD) or loader doesn’t support. Functionality such as Vulkan API tracing and debugging, API usage validation, and other tools such as framebuffer overlays are all natural candidates for Vulkan layers. Layers are implemented as libraries that are inserted between the application and the driver.
Not only is Vulkan a layered architecture but it also supports multiple GPUs and their drivers. Vulkan commands called by an application may wind up calling into a diverse set of modules: loader, layers, and ICDs. The loader is critical to managing the proper dispatching of Vulkan commands to the appropriate set of layers and ICDs. The Vulkan object model allows the loader to insert layers into a call chain so the layers can process Vulkan commands prior to the ICD being called.
Vulkan uses an object model to control the scope of a particular action / operation. The object to be acted on is always the first parameter of a Vulkan call and is a dispatchable object (see Vulkan specification section 2.2 Object Model). Under the covers, the dispatchable object handle is a pointer to a structure that contains a pointer to a dispatch table maintained by the loader. This dispatch table contains pointers to the Vulkan functions appropriate to that object. There are two types of dispatch tables the loader maintains, Instance and Device. I.e. a VkInstance object’s dispatch table will point to Vulkan functions such as vkEnumeratePhysicalDevices, vkDestroyInstance, vkCreateInstance, etc. Instance functions take a VkInstance or VkPhysicalDevice as their first argument.
Device objects have a separate dispatch table containing the appropriate function pointers. The device dispatch table is used for all functions that take a VkDevice, VkQueue or VkCommandBuffer as their first argument.
These instance and device dispatch tables are constructed when the application calls vkCreateInstance and vkCreateDevice. At that time the application and/or system can specify optional layers to be included. The loader will initialize the specified layers to create a call chain for each Vulkan function and each entry of the dispatch table will point to the first element of that chain. Thus, the loader builds an instance call chain for each VkInstance that is created and a device call chain for each VkDevice that is created.
For example, the diagram below represents what happens in the call chain for vkCreateInstance. After initializing the chain, the loader will call into the first layer’s vkCreateInstance which will call the next finally terminating in the loader again where this function calls every ICD’s vkCreateInstance and saves the results. This allows every enabled layer for this chain to set up what it needs based on the VkInstanceCreateInfo structure from the application.
This also highlights some of the complexity the loader must manage when using instance chains. As shown here, the loader must aggregate information from multiple devices when they are present. This means that the loader has to know about instance level extensions to aggregate them correctly.
Device chains are created at vkCreateDevice and are generally simpler because they deal with only a single device and the ICD can always be the terminator of the chain. The below diagram also illustrates how layers (either device or instance) can skip intercepting any given Vulkan entry point.
In this section we’ll discuss how an application interacts with the loader.
Linking to loader library for core and WSI extension symbols.
Dynamic Vulkan command lookup & application dispatch table.
Loader library filenames for linking to different Vulkan ABI versions.
Layers
Extensions
vkGetInstanceProcAddr, vkGetDeviceProcAddr
The loader library on Windows, Linux and Android will export all core Vulkan and all appropriate Window System Interface (WSI) extensions. This is done to make it simpler to get started with Vulkan development. When an application links directly to the loader library in this way, the Vulkan calls are simple trampoline functions that jump to the appropriate dispatch table entry for the object they are given.
Applications are not required to link directly to the loader library, instead they can use the appropriate platform specific dynamic symbol lookup on the loader library to initialize the application’s own dispatch table. This allows an application to fail gracefully if the loader cannot be found, and it provides the fastest mechanism for the application to call Vulkan functions. An application will only need to query (via system calls such as dlsym()) the address of vkGetInstanceProcAddr from the loader library. Using vkGetInstanceProcAddr the application can then discover the address of all instance and global functions and extensions, such as vkCreateInstance, vkEnumerateInstanceExtensionProperties and vkEnumerateInstanceLayerProperties in a platform independent way.
The Vulkan loader library will be distributed in various ways including Vulkan SDKs, OS package distributions and IHV driver packages. These details are beyond the scope of this document. However, the name and versioning of the Vulkan loader library is specified so an app can link to the correct Vulkan ABI library version. Vulkan versioning is such that ABI backwards compatibility is guaranteed for all versions with the same major number (e.g. 1.0 and 1.1). On Windows, the loader library encodes the ABI version in its name such that multiple ABI incompatible versions of the loader can peacefully coexist on a given system. The Vulkan loader library file name is “vulkan-<ABI version>.dll”. For example, for Vulkan version 1.X on Windows the library filename is vulkan-1.dll. And this library file can typically be found in the windows/system32 directory.
For Linux, shared libraries are versioned based on a suffix. Thus, the ABI number is not encoded in the base of the library filename as on Windows. On Linux an application wanting to link to the latest Vulkan ABI version would just link to the name vulkan (libvulkan.so). A specific Vulkan ABI version can also be linked to by applications (e.g. libvulkan.so.1).
Applications desiring Vulkan functionality beyond what the core API offers may use various layers or extensions. A layer cannot add new or modify existing Vulkan commands, but may offer extensions that do. A common use of layers is for API validation. A developer can use validation layers during application development, but during production the layers can be disabled by the application. Thus, eliminating the overhead of validating the application's usage of the API. Layers discovered by the loader can be reported to the application via vkEnumerateInstanceLayerProperties and vkEnumerateDeviceLayerProperties, for instance and device layers respectively. Instance layers are enabled at vkCreateInstance; device layers are enabled at vkCreateDevice. For example, the ppEnabledLayerNames array in the VkDeviceCreateInfo structure is used by the application to list the device layer names to be enabled at vkCreateDevice. At vkCreateInstance and vkCreateDevice, the loader will construct call chains that include the application specified (enabled) layers. Order is important in the ppEnabledLayerNames array; array element 0 is the topmost (closest to the application) layer inserted in the chain and the last array element is closest to the driver.
Developers may want to enable layers that are not enabled by the given application they are using. On Linux and Windows, the environment variables “VK_INSTANCE_LAYERS” and “VK_DEVICE_LAYERS” can be used to enable additional layers which are not specified (enabled) by the application at vkCreateInstance/vkCreateDevice. VK_INSTANCE_LAYERS is a colon (Linux)/semi-colon (Windows) separated list of layer names to enable. Order is relevant with the first layer in the list being the topmost layer (closest to the application) and the last layer in the list being the bottommost layer (closest to the driver).
Application specified layers and user specified layers (via environment variables) are aggregated and duplicates removed by the loader when enabling layers. Layers specified via environment variable are topmost (closest to the application) while layers specified by the application are bottommost.
An example of using these environment variables to activate the validation layer VK_LAYER_LUNARG_param_checker on Windows or Linux is as follows:
> $ export VK_INSTANCE_LAYERS=VK_LAYER_LUNARG_parameter_validation > $ export VK_DEVICE_LAYERS=VK_LAYER_LUNARG_parameter_validation
Note: Many layers, including all LunarG validation layers are “global” (i.e. both instance and device) layers and must be enabled on both the instance and device chains to function properly. This is required for “global” layers regardless of which method is used to enable the layer (application or environment variable).
Some platforms, including Linux and Windows, support layers which are enabled automatically by the loader rather than explicitly by the application (or via environment variable). Explicit layers are those layers enabled by the application (or environment variable) by providing the layer name. Implicit layers are those layers enabled by the loader automatically. Any implicit layers the loader discovers on the system in the appropriate location will be enabled (subject to environment variable overrides described later). Discovery of properly installed implicit and explicit layers is described later. Explicitly enabling a layer that is implicitly enabled has no additional effect: the layer will still be enabled implicitly by the loader.
Extensions are optional functionality provided by a layer, the loader or an ICD. Extensions can modify the behavior of the Vulkan API and need to be specified and registered with Khronos.
Instance extensions can be discovered via vkEnumerateInstanceExtensionProperties. Device extensions can be discovered via vkEnumerateDeviceExtensionProperties. The loader discovers and aggregates all extensions from layers (both explicit and implicit), ICDs and the loader before reporting them to the application in vkEnumerate*ExtensionProperties. The pLayerName parameter in these functions is used to select either a single layer or the Vulkan platform implementation. If pLayerName is NULL, extensions from Vulkan implementation components (including loader, implicit layers, and ICDs) are enumerated. If pLayerName is equal to a discovered layer module name then any extensions from that layer (which may be implicit or explicit) are enumerated. Duplicate extensions (e.g. an implicit layer and ICD might report support for the same extension) are eliminated by the loader. For duplicates, the ICD version is reported and the layer version is culled. Extensions must be enabled (in vkCreateInstance or vkCreateDevice) before they can be used.
Extension command entry points should be queried via vkGetInstanceProcAddr or vkGetDeviceProcAddr. vkGetDeviceProcAddr can only be used to query for device extension or core device entry points. Device entry points include any command that uses a VkDevice as the first parameter or a dispatchable object that is a child of a VkDevice (currently this includes VkQueue and VkCommandBuffer). vkGetInstanceProcAddr can be used to query either device or instance extension entry points in addition to all core entry points.
VkGetDeviceProcAddr is particularly interesting because it will provide the most efficient way to call into the ICD. For example, the diagram below shows what could happen if the application were to use vkGetDeviceProcAddr for the function “vkGetDeviceQueue” and “vkDestroyDevice” but not “vkAllocateMemory”. The resulting function pointer (fpGetDeviceQueue) would be the ICD’s entry point if the loader and any enabled layers do not need to see that call. Even if an enabled layer intercepts the call (e.g. vkDestroyDevice) the loader trampoline code is skipped for function pointers obtained via vkGetDeviceProcAddr. This also means that function pointers obtained via vkGetDeviceProcAddr will only work with the specific VkDevice it was created for, using it with another device has undefined results. For extensions, Get*ProcAddr will often be the only way to access extension API features.
Vulkan allows multiple drivers each with one or more devices (represented by a Vulkan VkPhysicalDevice object) to be used collectively. The loader is responsible for discovering available Vulkan ICDs on the system. Given a list of available ICDs, the loader can enumerate all the physical devices available for an application and return this information to the application. The process in which the loader discovers the available Installable Client Drivers (ICDs) on a system is platform dependent. Windows, Linux and Android ICD discovery details are listed below.
In order to find properly-installed ICDs, the Vulkan loader will scan the values in the following Windows registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Khronos\Vulkan\Drivers
For each value in this key which has DWORD data set to 0, the loader opens the JSON format text information file (a.k.a. "manifest file") specified by the name of the value. Each name must be a full pathname to the text manifest file. The Vulkan loader will open each manifest file to obtain the name or pathname of an ICD shared library (".dll") file. For example:
{ "file_format_version": "1.0.0", "ICD": { "library_path": "path to ICD library", "api_version": "1.0.5" } }
The "library_path" specifies either a filename, a relative pathname, or a full pathname to an ICD shared library file, which the loader will attempt to load using LoadLibrary(). If the ICD is specified via a filename, the shared library lives in the system's DLL search path (e.g. in the "C:\Windows\System32" folder). If the ICD is specified via a relative pathname, it is relative to the path of the manifest file. Relative pathnames are those that do not start with a drive specifier (e.g. "C:"), nor with a directory separator (i.e. the '\' character), but do contain at least one directory separator.
The "file_format_version" specifies a major.minor.patch version number in case the format of the text information file changes in the future. If the same ICD shared library supports multiple, incompatible versions of text manifest file format versions, it must have multiple text info files (all of which may point to the same shared library).
The “api_version” specifies the major.minor.patch version number of the Vulkan API that the shared library (referenced by "library_path") was built with.
There are no rules about the name of the text information files (except the .json suffix).
There are no rules about the name of the ICD shared library files. For example, if the registry contains the following values,
[HKEY_LOCAL_MACHINE\SOFTWARE\Khronos\Vulkan\Drivers\] "C:\vendor a\vk_vendora.json"=dword:00000000 "C:\windows\system32\vendorb_vk.json"=dword:00000000 "C:\windows\system32\vendorc_icd.json"=dword:00000000
then the loader will open the following text information files, with the specified contents:
Text File Name | Text File Contents |
---|---|
vk_vendora.json | "ICD": { "library_path": "C:\VENDOR A\vk_vendora.dll", "api_version": "1.0.5" } |
vendorb_vk.json | "ICD": { "library_path": "vendorb_vk.dll", "api_version": "1.0.5" } |
vendorc_icd.json | "ICD": { "library_path": "vedorc_icd.dll", "api_version": "1.0.5" } |
Then the loader will open the three files mentioned in the "Text File Contents" column, and then try to load and use the three shared libraries indicated by the ICD.library_path value.
IHV developers (and sometimes other developers) need to use special, pre-production ICDs. In some cases, a pre-production ICD may be in an installable package. In other cases, a pre-production ICD may simply be a shared library in the developer's build tree. In this latter case, we want to allow developers to point to such an ICD without modifying the properly-installed ICD(s) on their system.
This need is met with the use of the "VK_ICD_FILENAMES" environment variable, which will override the mechanism used for finding properly-installed ICDs. In other words, only the ICDs listed in "VK_ICD_FILENAMES" will be used. The "VK_ICD_FILENAMES" environment variable is a semi-colon-separated list of ICD text information files (aka manifest files), containing the following:
Typically, "VK_ICD_FILENAMES" will only contain a full pathname to one info file for a developer-built ICD. A semi-colon is only used if more than one ICD is listed.
For example, if a developer wants to refer to one ICD that they built, they could set the "VK_ICD_FILENAMES" environment variable to:
C:\my_build\my_icd.json
If a developer wants to refer to two ICDs, one of which is a properly-installed ICD, they can use the full pathname of the text file:
C:\Windows\System32\vendorc_icd.json;C:\my_build\my_icd.json
Notice the semi-colon between "C:\Windows\System32\vendorc_icd.json" and "C:\my_build\my_icd.json".
In order to find properly-installed ICDs, the Vulkan loader will scan the files in the following Linux directories:
/usr/share/vulkan/icd.d /etc/vulkan/icd.d $HOME/.local/share/vulkan/icd.d
Where $HOME is the current home directory of the application's user id; this path will be ignored for suid programs.
These directories will contain text information files (a.k.a. "manifest files"), that use a JSON format.
The Vulkan loader will open each manifest file found to obtain the name or pathname of an ICD shared library (".so") file. For example:
{ "file_format_version": "1.0.0", "ICD": { "library_path": "path to ICD library", "api_version": "1.0.5" } }
The "library_path" specifies either a filename, a relative pathname, or a full pathname to an ICD shared library file. If the ICD is specified via a filename, the loader will attempt to open that file as a shared object using dlopen(), and the file must be in a directory that dlopen is configured to look in (Note: various distributions are configured differently). A distribution is free to create Vulkan-specific system directories (e.g. ".../vulkan/icd"), but is not required to do so. If the ICD is specified via a relative pathname, it is relative to the path of the info file. Relative pathnames are those that do not start with, but do contain at least one directory separator (i.e. the '/' character). For example, "lib/vendora.so" and "./vendora.so" are examples of relative pathnames.
The "file_format_version" provides a major.minor.patch version number in case the format of the manifest file changes in the future. If the same ICD shared library supports multiple, incompatible versions of manifest file format versions, it must have multiple manifest files (all of which may point to the same shared library).
The “api_version” specifies the major.minor.patch version number of the Vulkan API that the shared library (referenced by "library_path") was built with.
The "/usr/share/vulkan/icd.d" directory is for ICDs that are installed from Linux-distribution-provided packages. The "/etc/vulkan/icd.d" directory is for ICDs that are installed from non-Linux-distribution-provided packages.
There are no rules about the name of the text files (except the .json suffix).
There are no rules about the name of the ICD shared library files. For example, if the "/usr/share/vulkan/icd.d" directory contain the following files, with the specified contents:
Text File Name | Text File Contents |
---|---|
vk_vendora.json | "ICD": { "library_path": "vendora.so", "api_version": "1.0.5" } |
vendorb_vk.json | "ICD": { "library_path": "vendorb_vulkan_icd.so", "api_version": "1.0.5" } |
vendorc_icd.json | "ICD": { "library_path": "/usr/lib/VENDORC/icd.so", "api_version": "1.0.5" } |
then the loader will open the three files mentioned in the "Text File Contents" column, and then try to load and use the three shared libraries indicated by the ICD.library_path value.
IHV developers (and sometimes other developers) need to use special, pre-production ICDs. In some cases, a pre-production ICD may be in an installable package. In other cases, a pre-production ICD may simply be a shared library in the developer's build tree. In this latter case, we want to allow developers to point to such an ICD without modifying the properly-installed ICD(s) on their system.
This need is met with the use of the "VK_ICD_FILENAMES" environment variable, which will override the mechanism used for finding properly-installed ICDs. In other words, only the ICDs listed in "VK_ICD_FILENAMES" will be used.
The "VK_ICD_FILENAMES" environment variable is a colon-separated list of ICD manifest files, containing the following:
A filename (e.g. "libvkicd.json") in the "/usr/share/vulkan/icd.d", "/etc/vulkan/icd.d" "$HOME/.local/share/vulkan/icd.d" directories
A full pathname (e.g. "/my_build/my_icd.json")
Typically, "VK_ICD_FILENAMES" will only contain a full pathname to one info file for a developer-built ICD. A colon is only used if more than one ICD is listed.
For example, if a developer wants to refer to one ICD that they built, they could set the "VK_ICD_FILENAMES" environment variable to:
/my_build/my_icd.json
If a developer wants to refer to two ICDs, one of which is a properly-installed ICD, they can use the name of the text file in the system directory:
vendorc_vulkan.json:/my_build/my_icd.json
Notice the colon between "vendorc_vulkan.json" and "/my_build/my_icd.json".
NOTE: this environment variable will be ignored for suid programs.
The Android loader lives in the system library folder. The location cannot be changed. The loader will load the driver/ICD via hw_get_module with the ID of "vulkan". Due to security policies in Android none of this can be modified under normal use.
Generally, for all Vulkan commands issued by an application, the loader can be viewed as a pass through. That is, the loader generally doesn’t modified the commands or their parameters but simply calls the ICDs entry point for that command. Thus, the loader to ICD interface requirements will be specified by covering two areas: 1) Obtaining ICD Vulkan entry points; 2) Specifying requirements for a given Vulkan command(s) over and above the Vulkan specification requirements.
Currently, two methods of the loader finding ICD entry points are supported on Linux and Windows:
vk_icdGetInstanceProcAddr is exported by the ICD library and it returns valid function pointers for all the global level and instance level Vulkan commands, and also for vkGetDeviceProcAddr. Global level commands are those which contain no dispatchable object as the first parameter, such as vkCreateInstance and vkEnumerateInstanceExtensionProperties. The ICD must support querying global level entry points by calling vk_icdGetInstanceProcAddr with a NULL VkInstance parameter. Instance level commands are those that have either VkInstance, or VkPhysicalDevice as the first parameter dispatchable object. Both core entry points and any instance extension entry points the ICD supports should be available via vk_icdGetInstanceProcAddr. Future Vulkan instance extensions may define and use new instance level dispatchable objects other than VkInstance and VkPhysicalDevice, in which case extension entry points using these newly defined dispatchable objects must be queryable via vk_icdGetInstanceProcAddr.
All other Vulkan entry points must either NOT be exported from the ICD library or else NOT use the official Vulkan function names if they are exported. This requirement is for ICD libraries that include other functionality (such as OpenGL library) and thus could be loaded by the application prior to when the Vulkan loader library is loaded by the application. In other words, the ICD library exported Vulkan symbols must not clash with the loader's exported Vulkan symbols.
Beware of interposing by dynamic OS library loaders if the official Vulkan names are used. On Linux, if official names are used, the ICD library must be linked with -Bsymbolic.
vkGetInstanceProcAddr exported in the ICD library and returns valid function pointers for all the Vulkan API entry points.
vkCreateInstance exported in the ICD library;
vkEnumerateInstanceExtensionProperties exported in the ICD library;
Normally, ICDs handle object creation and destruction for various Vulkan objects. The WSI surface extensions for Linux and Windows (VK_KHR_win32_surface, VK_KHR_xcb_surface, VK_KHR_xlib_surface, VK_KHR_mir_surface, VK_KHR_wayland_surface, and VK_KHR_surface) are handled differently. For these extensions, the VkSurfaceKHR object creation and destruction is handled by the loader as follows:
Loader handles the vkCreate*SurfaceKHR() and vkDestroySurfaceKHR() functions including creating/destroying the VkSurfaceKHR object.
VkSurfaceKHR objects have the underlying structure (VkIcdSurface*) as defined in include/vulkan/vk_icd.h.
ICDs can cast any VkSurfaceKHR object to a pointer to the appropriate VkIcdSurface* structure.
VkIcdSurface* structures include VkIcdSurfaceWin32, VkIcdSurfaceXcb, VkIcdSurfaceXlib, VkIcdSurfaceMir, and VkIcdSurfaceWayland. The first field in the structure is a VkIcdSurfaceBase enumerant that indicates whether the surface object is Win32, Xcb, Xlib, Mir, or Wayland.
As previously covered, the loader requires dispatch tables to be accessible within Vulkan dispatchable objects, which include VkInstance, VkPhysicalDevice, VkDevice, VkQueue, and VkCommandBuffer. The specific requirements on all dispatchable objects created by ICDs are as follows:
All dispatchable objects created by an ICD can be cast to void **
The loader will replace the first entry with a pointer to the dispatch table which is owned by the loader. This implies three things for ICD drivers:
The ICD must return a pointer for the opaque dispatchable object handle.
This pointer points to a regular C structure with the first entry being a pointer. Note: for any C++ ICD's that implement VK objects directly as C++ classes. The C++ compiler may put a vtable at offset zero if your class is non-POD due to the use of a virtual function. In this case use a regular C structure (see below).
The loader checks for a magic value (ICD_LOADER_MAGIC) in all the created dispatchable objects, as follows (see include/vulkan/vk_icd.h):
#include "vk_icd.h" union _VK_LOADER_DATA { uintptr loadermagic; void *loaderData; } VK_LOADER_DATA; vkObj alloc_icd_obj() { vkObj *newObj = alloc_obj(); ... // Initialize pointer to loader's dispatch table with ICD_LOADER_MAGIC set_loader_magic_value(newObj); ... return newObj; }
Additional Notes:
The Android loader uses the same protocol for initializing the dispatch table as described above. The only difference is that the Android loader queries layer and extension information directly from the respective libraries and does not use the json manifest files used by the Windows and Linux loaders.
In order to find properly-installed layers, the Vulkan loader will use a similar mechanism as used for ICDs. Text information files (aka manifest files), that use a JSON format, are read in order to identify the names and attributes of layers and their extensions. The use of manifest files allows the loader to avoid loading any shared library files when the application does not query nor request any extensions. Layers and extensions have additional complexity, and so their manifest files contain more information than ICD info files. For example, a layer shared library file may contain multiple layers/extensions (perhaps even an ICD).
In order to find properly-installed layers, the Vulkan loader will scan the values in the following Windows registry keys:
HKEY_LOCAL_MACHINE\SOFTWARE\Khronos\Vulkan\ExplicitLayers
HKEY_LOCAL_MACHINE\SOFTWARE\Khronos\Vulkan\ImplicitLayers
Explicit layers are those which are enabled by an application (e.g. with the vkCreateInstance function), or by an environment variable (as mentioned previously).
Implicit layers are those which are enabled by their existence. For example, certain application environments (e.g. Steam or an automotive infotainment system) may have layers which they always want enabled for all applications that they start. Other implicit layers may be for all applications started on a given system (e.g. layers that overlay frames-per-second). Implicit layers are enabled automatically, whereas explicit layers must be enabled explicitly. What distinguishes a layer as implicit or explicit is by which registry key its layer information file is referenced by.
For each value in these keys which has DWORD data set to 0, the loader opens the JSON manifest file specified by the name of the value. Each name must be a full pathname to the manifest file.
The Vulkan loader will open each info file to obtain information about the layer, including the name or pathname of a shared library (".dll") file.
This manifest file is in the JSON format and contains the following information:
(required) "file_format_version" - same as for ICDs, except that the format version can vary independently for ICDs and layers.
(required) "name" - layer name
(required) "type" - which layer chains should the layer be activated on. Allowable values are "INSTANCE", "DEVICE", "GLOBAL". Global means activate on both device and instance chains.
(required) "library_path" - filename / full path / relative path to the library file
(required) "api_version" - same as for ICDs.
(required) "implementation_version" - layer version, a single number increasing with backward compatible changes.
(required) "description" - informative description of the layer.
(optional) "device_extensions" or "instance_extensions" - array of extension information as follows
(required) extension "name" - Vulkan registered name
(required) extension "spec_version" - extension specification version, a single number, increasing with backward compatible changes.
(required for device_extensions with entry points) extension "entrypoints" - array of device extension entry points; not used for instance extensions
(sometimes required) "functions" - mapping list of function entry points. If multiple layers exist within the same shared library (or if a layer is in the same shared library as an ICD), this must be specified to allow each layer to have its own vkGet*ProcAddr entry points that can be found by the loader. At this time, only the following two functions are required:
"vkGetInstanceProcAddr" name
"vkGetDeviceProcAddr" name
(optional for implicit layers) "enable_environment" requirement(s) - environment variable and value required to enable an implicit layer. This environment variable (which should vary with each "version" of the layer, as in "ENABLE_LAYER_FOO_1") must be set to the given value or else the implicit layer is not loaded. This is for application environments (e.g. Steam) which want to enable a layer(s) only for applications that they launch, and allows for applications run outside of an application environment to not get that implicit layer(s).
(required for implicit layers) "disable_environment" requirement(s) - environment variable and value required to disable an implicit layer. Note: in rare cases of an application not working with an implicit layer, the application can set this environment variable (before calling Vulkan functions) in order to "blacklist" the layer. This environment variable (which should vary with each "version" of the layer, as in "DISABLE_LAYER_FOO_1") must be set (not particularly to any value). If both the "enable_environment" and "disable_environment" variables are set, the implicit layer is disabled.
For example:
{ "file_format_version" : "1.0.0", "layer": { "name": "VK_LAYER_LUNARG_OverlayLayer", "type": "DEVICE", "library_path": "vkOverlayLayer.dll" "api_version" : "1.0.5", "implementation_version" : "2", "description" : "LunarG HUD layer", "functions": { "vkGetInstanceProcAddr": "OverlayLayer_GetInstanceProcAddr", "vkGetDeviceProcAddr": "OverlayLayer_GetDeviceProcAddr" }, "instance_extensions": [ { "name": "VK_debug_report_EXT", "spec_version": "1" }, { "name": "VK_VENDOR_DEBUG_X", "spec_version": "3" } ], "device_extensions": [ { "name": "VK_DEBUG_MARKER_EXT", "spec_version": "1", "entrypoints": ["vkCmdDbgMarkerBegin", "vkCmdDbgMarkerEnd"] } ], "disable_environment": { "DISABLE_LAYER_OVERLAY_1": "" } } }
The "library_path" specifies either a filename, a relative pathname, or a full pathname to a layer shared library (".dll") file, which the loader will attempt to load using LoadLibrary(). If the layer is specified via a relative pathname, it is relative to the path of the info file (e.g. for cases when an application provides a layer that is in the same folder hierarchy as the rest of the application files). If the layer is specified via a filename, the shared library lives in the system's DLL search path (e.g. in the "C:\Windows\System32" folder).
There are no rules about the name of the text files (except the .json suffix).
There are no rules about the name of the layer shared library files.
As with ICDs, developers may need to use special, pre-production layers, without modifying the properly-installed layers. This need is met with the use of the "VK_LAYER_PATH" environment variable, which will override the mechanism using for finding properly-installed layers. Because many layers may exist on a system, this environment variable is a semi-colon-separated list of folders that contain layer info files. Only the folder listed in "VK_LAYER_PATH" will be scanned for info files. Each semi-colon-separated entry is:
In order to find properly-installed layers, the Vulkan loader will use a similar mechanism as used for ICDs. Text information files, that use a JSON format, are read in order to identify the names and attributes of layers and their extensions. The use of text info files allows the loader to avoid loading any shared library files when the application does not query nor request any extensions. Layers and extensions have additional complexity, and so their info files contain more information than ICD info files. For example, a layer shared library file may contain multiple layers/extensions (perhaps even an ICD).
The Vulkan loader will scan the files in the following Linux directories:
/usr/share/vulkan/explicit_layer.d /usr/share/vulkan/implicit_layer.d /etc/vulkan/explicit_layer.d /etc/vulkan/implicit_layer.d $HOME/.local/share/vulkan/explicit_layer.d $HOME/.local/share/vulkan/implicit_layer.d
Where $HOME is the current home directory of the application's user id; this path will be ignored for suid programs.
Explicit layers are those which are enabled by an application (e.g. with the vkCreateInstance function), or by an environment variable (as mentioned previously). Implicit layers are those which are enabled by their existence. For example, certain application environments (e.g. Steam or an automotive infotainment system) may have layers which they always want enabled for all applications that they start. Other implicit layers may be for all applications started on a given system (e.g. layers that overlay frames-per-second). Implicit layers are enabled automatically, whereas explicit layers must be enabled explicitly. What distinguishes a layer as implicit or explicit is by which directory its layer information file exists in.
The "/usr/share/vulkan/*_layer.d" directories are for layers that are installed from Linux-distribution-provided packages. The "/etc/vulkan/*_layer.d" directories are for layers that are installed from non-Linux-distribution-provided packages.
The information file is in the JSON format and contains the following information:
(required) "file_format_version" – same as for ICDs, except that the format version can vary independently for ICDs and layers.
(required) "name" - layer name
(required) "type" - which layer chains should the layer be activated on. Allowable values are "INSTANCE", "DEVICE", "GLOBAL". Global means activate on both device and instance chains.
(required) "library_path" - filename / full path / relative path to the text file
(required) "api_version" – same as for ICDs.
(required) "implementation_version" – layer version, a single number increasing with backward compatible changes.
(required) "description" – informative description of the layer.
(optional) "device_extensions" or "instance_extensions" - array of extension information as follows
(required) extension "name" - Vulkan registered name
(required) extension "spec_version" - extension specification version, a single number, increasing with backward compatible changes.
(required for device extensions with entry points) extension "entrypoints" - array of device extension entry points; not used for instance extensions
(sometimes required) "functions" - mapping list of function entry points. If multiple layers exist within the same shared library (or if a layer is in the same shared library as an ICD), this must be specified to allow each layer to have its own vkGet*ProcAddr entry points that can be found by the loader. At this time, only the following two functions are required:
(optional for implicit layers) "enable_environment" requirement(s) - environment variable and value required to enable an implicit layer. This environment variable (which should vary with each "version" of the layer, as in "ENABLE_LAYER_FOO_1") must be set to the given value or else the implicit layer is not loaded. This is for application environments (e.g. Steam) which want to enable a layer(s) only for applications that they launch, and allows for applications run outside of an application environment to not get that implicit layer(s).
(required for implicit layers) "disable_environment" requirement(s) - environment variable and value required to disable an implicit layer. Note: in rare cases of an application not working with an implicit layer, the application can set this environment variable (before calling Vulkan functions) in order to "blacklist" the layer. This environment variable (which should vary with each "version" of the layer, as in "DISABLE_LAYER_FOO_1") must be set (not particularly to any value). If both the "enable_environment" and "disable_environment" variables are set, the implicit layer is disabled.
For example:
{ "file_format_version" : "1.0.0", "layer": { "name": "VK_LAYER_LUNARG_OverlayLayer", "type": "DEVICE", "library_path": "vkOverlayLayer.dll" "api_version" : "1.0.5", "implementation_version" : "2", "description" : "LunarG HUD layer", "functions": { "vkGetInstanceProcAddr": "OverlayLayer_GetInstanceProcAddr", "vkGetDeviceProcAddr": "OverlayLayer_GetDeviceProcAddr" }, "instance_extensions": [ { "name": "VK_debug_report_EXT", "spec_version": "1" }, { "name": "VK_VENDOR_DEBUG_X", "spec_version": "3" } ], "device_extensions": [ { "name": "VK_DEBUG_MARKER_EXT", "spec_version": "1", "entrypoints": ["vkCmdDbgMarkerBegin", "vkCmdDbgMarkerEnd"] } ], "disable_environment": { "DISABLE_LAYER_OVERLAY_1": "" } } }
The "library_path" specifies either a filename, a relative pathname, or a full pathname to a layer shared library (".so") file, which the loader will attempt to load using dlopen(). If the layer is specified via a filename, the loader will attempt to open that file as a shared object using dlopen(), and the file must be in a directory that dlopen is configured to look in (Note: various distributions are configured differently). A distribution is free to create Vulkan-specific system directories (e.g. ".../vulkan/layers"), but is not required to do so. If the layer is specified via a relative pathname, it is relative to the path of the info file (e.g. for cases when an application provides a layer that is in the same directory hierarchy as the rest of the application files).
There are no rules about the name of the text files (except the .json suffix).
There are no rules about the name of the layer shared library files.
As with ICDs, developers may need to use special, pre-production layers, without modifying the properly-installed layers. This need is met with the use of the "VK_LAYER_PATH" environment variable, which will override the mechanism using for finding properly-installed layers. Because many layers may exist on a system, this environment variable is a colon-separated list of directories that contain layer info files. Only the directories listed in "VK_LAYER_PATH" will be scanned for info files. Each colon-separated entry is:
NOTE: these environment variables will be ignored for suid programs.
The recommended way to enable layers is for applications to programatically enable them. The layers are provided by the application and must live in the application's library folder. The application enables the layers at vkCreateInstance and vkCreateDevice as any Vulkan application would. An application enabled for debug has more options. It can enumerate and enable layers located in /data/local/vulkan/debug.
There are two key architectural features that drive the loader to layer library interface: 1) separate and distinct instance and device call chains, and 2) distributed dispatch. First these architectural features will be described and then the detailed interface will be specified.
Call chains are the links of calls for a given Vulkan command from layer module to layer module with the loader and or the ICD being the bottom most command. Call chains are constructed at both the instance level and the device level by the loader with cooperation from the layer libraries. Instance call chains are constructed by the loader when layers are enabled at vkCreateInstance. Device call chains are constructed by the loader when layers are enabled at vkCreateDevice. A layer can intercept Vulkan instance commands, device commands or both. For a layer to intercept instance commands, it must participate in the instance call chain. For a layer to intercept device commands, it must participate in the device chain. Layers which participate in intercepting calls in both the instance and device chains are called global layers.
Normally, when a layer intercepts a given Vulkan command, it will call down the instance or device chain as needed. The loader and all layer libraries that participate in a call chain cooperate to ensure the correct sequencing of calls from one entity to the next. This group effort for call chain sequencing is hereinafter referred to as distributed dispatch. In distributed dispatch, since each layer is responsible for properly calling the next entity in the device or instance chain, a dispatch mechanism is required for all Vulkan commands a layer intercepts. For Vulkan commands that are not intercepted by a layer, or if the layer chooses to terminate a given Vulkan command by not calling down the chain, then no dispatch mechanism is needed for that particular Vulkan command. Only for those Vulkan commands, which may be a subset of all Vulkan commands, that a layer intercepts is a dispatching mechanism by the layer needed. The loader is responsible for dispatching all core and instance extension Vulkan commands to the first entity in the chain.
Instance level Vulkan commands are those that have the dispatchable objects VkInstance, or VkPhysicalDevice as the first parameter and also includes vkCreateInstance.
Device level Vulkan commands are those that use VkDevice, VkQueue or VkCommandBuffer as the first parameter and also include vkCreateDevice. Future extensions may introduce new instance or device level dispatchable objects, so the above lists may be extended in the future.
A layer library is a container of layers. This section defines an extensible programming interface to discover layers contained in layer libraries. It also specifies the minimal conventions and rules a layer must follow.
Other sections might have other guidelines that layers, at least validation layers, should follow.
A layer, when inserted into an otherwise compliant Vulkan implementation, must still result in a compliant Vulkan implementation[*]. It must additionally follow some conventions and rules.
A layer is always chained with other layers. It must not make invalid calls to or rely on undefined behaviors of its lower layers. When it changes the behavior of a command, it must make sure its upper layers do not make invalid calls to or rely on undefined behaviors of its lower layers because of the changed behavior. For example, when a layer intercepts an object creation command to wrap the objects created by its lower layers, it must make sure its lower layers never see the wrapping objects, directly from itself or indirectly from its upper layers.
When a layer requires host memory, it is free to scope the allocations to itself, bypassing the provided allocators entirely.
vkEnumerateDeviceExtensionProperties
must handle the case where pLayerName
is NULL
, usually by chaining to other layers.
vkGetInstanceProcAddr
can intercept a command by returning a function pointer different from what would be returned through chaining.
vkGetDeviceProcAddr
can intercept a command by returning a function pointer different from what would be returned through chaining.
vkCreateInstance
must not generate an error for unrecognized layer names, extension names, and pNext
structs. It may assume the layer names and extension names have been validated.
vkCreateDevice
must not generate an error for unrecognized layer names, extension names, and pNext
structs. It may assume the layer names and extension names have been validated.
[*]: The intention is for layers to have a well-defined baseline behavior. Some of the conventions or rules, for example, may be considered abuses of the specification.
A layer library supporting interface version 0 must define and export these functions, unrelated to any Vulkan command despite the names, signatures, and other similarities:
vkEnumerateInstanceLayerProperties
enumerates all instance layers in a layer library. This function never fails.
vkEnumerateInstanceExtensionProperties
enumerates instance extensions of instance layers in a layer library. pLayerName
is always a valid instance layer name. This function never fails.
vkEnumerateDeviceLayerProperties
enumerates all device layers in a layer library. physicalDevice
is always VK_NULL_HANDLE
. This function never fails.
vkEnumerateDeviceExtensionProperties
enumerates device extensions of device layers in a layer library. physicalDevice
is always VK_NULL_HANDLE
. pLayerName
is always a valid device layer name. This function never fails.
<layerName>GetInstanceProcAddr
behaves as if <layerName>
's vkGetInstanceProcAddr
is called, except
pName
is vkEnumerateInstanceLayerProperties
, vkEnumerateInstanceExtensionProperties
, or vkEnumerateDeviceLayerProperties
(but not vkEnumerateDeviceExtensionProperties
), it returns a function pointer to the function of the same name defined by this interface.pName
is vkGetInstanceProcAddr
, it returns a function pointer to itself.pName
is vkCreateDevice
, it ignores instance
.pName
is a device command defined by Vulkan 1.0 or VK_KHR_swapchain
(but not other device commands), it may chain to other layers without intercepting. A loader should avoid querying such device commands.When a layer library contains only one layer, this function may alternatively be named vkGetInstanceProcAddr
.
<layerName>GetDeviceProcAddr
behaves as if <layerName>
's vkGetDeviceProcAddr
is called.
When a layer library contains only one layer, this function may alternatively be named vkGetDeviceProcAddr
.
All contained layers must support vk_layer.h
. They do not need to implement commands that are not queryable. They are recommended not to export any command.
The layer libraries and the manifest files must be kept in sync. On Windows and Linux, the loader uses manifest files to discover layer libraries and layers. On Android, the loader inspects the layer libraries directly.
The manifest file specifies in its "functions", "vkGetInstanceProcAddr" node the function name the loader will use to discover GetInstanceProcAddr. When the layer library exports GetInstanceProcAddr, the JSON node must explicitly specify that function name. When the layer library exports vkGetInstanceProcAddr, the JSON node may be omitted. The layer library may additionally export another alias and specify that in the JSON node.
Similarly, the manifest file specifies in its "functions", "vkGetDeviceProcAddr" node the function name the loader will use to discover GetDeviceProcAddr. When the layer library exports GetDeviceProcAddr, the JSON node must explicitly specify that function name. When the layer library exports vkGetDeviceProcAddr, the JSON node may be omitted. The layer library may additionally export another alias and specify that in the JSON node.
VkResult vkCreateInstance( const VkInstanceCreateInfo *pCreateInfo, const VkAllocationCallbacks *pAllocator, VkInstance *pInstance) { VkLayerInstanceCreateInfo *chain_info = get_chain_info(pCreateInfo, VK_LAYER_LINK_INFO); assert(chain_info->u.pLayerInfo); PFN_vkGetInstanceProcAddr fpGetInstanceProcAddr = chain_info->u.pLayerInfo->pfnNextGetInstanceProcAddr; PFN_vkCreateInstance fpCreateInstance = (PFN_vkCreateInstance)fpGetInstanceProcAddr(NULL, "vkCreateInstance"); if (fpCreateInstance == NULL) { return VK_ERROR_INITIALIZATION_FAILED; } // Advance the link info for the next element of the chain chain_info->u.pLayerInfo = chain_info->u.pLayerInfo->pNext; // Continue call down the chain VkResult result = fpCreateInstance(pCreateInfo, pAllocator, pInstance); if (result != VK_SUCCESS) return result; // Init layer's dispatch table using GetInstanceProcAddr of // next layer in the chain. instance_dispatch_table = new VkLayerInstanceDispatchTable; layer_init_instance_dispatch_table( *pInstance, my_data->instance_dispatch_table, fpGetInstanceProcAddr); // Other layer initialization ... return VK_SUCCESS; }
VkResult vkCreateDevice( VkPhysicalDevice gpu, const VkDeviceCreateInfo *pCreateInfo, const VkAllocationCallbacks *pAllocator, VkDevice *pDevice) { VkLayerDeviceCreateInfo *chain_info = get_chain_info(pCreateInfo, VK_LAYER_LINK_INFO); PFN_vkGetInstanceProcAddr fpGetInstanceProcAddr = chain_info->u.pLayerInfo->pfnNextGetInstanceProcAddr; PFN_vkGetDeviceProcAddr fpGetDeviceProcAddr = chain_info->u.pLayerInfo->pfnNextGetDeviceProcAddr; PFN_vkCreateDevice fpCreateDevice = (PFN_vkCreateDevice)fpGetInstanceProcAddr(NULL, "vkCreateDevice"); if (fpCreateDevice == NULL) { return VK_ERROR_INITIALIZATION_FAILED; } // Advance the link info for the next element on the chain chain_info->u.pLayerInfo = chain_info->u.pLayerInfo->pNext; VkResult result = fpCreateDevice(gpu, pCreateInfo, pAllocator, pDevice); if (result != VK_SUCCESS) { return result; } // initialize layer's dispatch table device_dispatch_table = new VkLayerDispatchTable; layer_init_device_dispatch_table( *pDevice, device_dispatch_table, fpGetDeviceProcAddr); // Other layer initialization ... return VK_SUCCESS; }
A layer may want to associate it's own private data with one or more Vulkan objects. Two common methods to do this are hash maps and object wrapping. The loader supports layers wrapping any Vulkan object including dispatchable objects. Layers which wrap objects should ensure they always unwrap objects before passing them down the chain. This implies the layer must intercept every Vulkan command which uses the object in question. Layers above the object wrapping layer will see the wrapped object. Layers which wrap dispatchable objects must ensure that the first field in the wrapping structure is a pointer to a dispatch table as defined in vk_layer.h. Specifically, an instance wrapped dispatchable object could be as follows:
struct my_wrapped_instance_obj_ { VkLayerInstanceDispatchTable *disp; // whatever data layer wants to add to this object };
A device wrapped dispatchable object could be as follows:
struct my_wrapped_instance_obj_ { VkLayerDispatchTable *disp; // whatever data layer wants to add to this object };
Alternatively, a layer may want to use a hash map to associate data with a given object. The key to the map could be the object. Alternatively, for dispatchable objects at a given level (eg device or instance) the layer may want data associated with the VkDevice or VkInstance objects. Since there are multiple dispatchable objects for a given VkInstance or VkDevice, the VkDevice or VkInstance object is not a great map key. Instead the layer should use the dispatch table pointer within the VkDevice or VkInstance since that will be unique for a given VkInstance or VkDevice.
Layers which create dispatchable objects take special care. Remember that loader trampoline code normally fills in the dispatch table pointer in the newly created object. Thus, the layer must fill in the dispatch table pointer if the loader trampoline will not do so. Common cases where a layer (or ICD) may create a dispatchable object without loader trampoline code is as follows:
The Windows/Linux loader provides a callback that can be used for initializing a dispatchable object. The callback is passed as an extension structure via the pNext field in VkInstanceCreateInfo and VkDeviceCreateInfo. The callback prototype is defined as follows for instance and device callbacks respectively (see vk_layer.h):
VKAPI_ATTR VkResult VKAPI_CALL vkSetInstanceLoaderData(VkInstance instance, void *object); VKAPI_ATTR VkResult VKAPI_CALL vkSetDeviceLoaderData)(VkDevice device, void *object);
To obtain these callbacks the layer must search through the list of structures pointed to by the "pNext" field in the VkInstanceCreateInfo and VkDeviceCreateInfo parameters to find any callback structures inserted by the loader. The salient details are as follows:
Alternatively, if an older loader is being used that doesn't provide these callbacks, the layer may manually initialize the newly created dispatchable object. To fill in the dispatch table pointer in newly created dispatchable object, the layer should copy the dispatch pointer, which is always the first entry in the structure, from an existing parent object of the same level (instance versus device). For example, if there is a newly created VkCommandBuffer object, then the dispatch pointer from the VkDevice object, which is the parent of the VkCommandBuffer object, should be copied into the newly created object.