lguest: documentation II: Guest
Documentation: The Guest
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/lguest.h b/include/linux/lguest.h
index 500aace..e76c151 100644
--- a/include/linux/lguest.h
+++ b/include/linux/lguest.h
@@ -27,18 +27,38 @@
#define LG_CLOCK_MIN_DELTA 100UL
#define LG_CLOCK_MAX_DELTA ULONG_MAX
+/*G:031 First, how does our Guest contact the Host to ask for privileged
+ * operations? There are two ways: the direct way is to make a "hypercall",
+ * to make requests of the Host Itself.
+ *
+ * Our hypercall mechanism uses the highest unused trap code (traps 32 and
+ * above are used by real hardware interrupts). Seventeen hypercalls are
+ * available: the hypercall number is put in the %eax register, and the
+ * arguments (when required) are placed in %edx, %ebx and %ecx. If a return
+ * value makes sense, it's returned in %eax.
+ *
+ * Grossly invalid calls result in Sudden Death at the hands of the vengeful
+ * Host, rather than returning failure. This reflects Winston Churchill's
+ * definition of a gentleman: "someone who is only rude intentionally". */
#define LGUEST_TRAP_ENTRY 0x1F
static inline unsigned long
hcall(unsigned long call,
unsigned long arg1, unsigned long arg2, unsigned long arg3)
{
+ /* "int" is the Intel instruction to trigger a trap. */
asm volatile("int $" __stringify(LGUEST_TRAP_ENTRY)
+ /* The call is in %eax (aka "a"), and can be replaced */
: "=a"(call)
+ /* The other arguments are in %eax, %edx, %ebx & %ecx */
: "a"(call), "d"(arg1), "b"(arg2), "c"(arg3)
+ /* "memory" means this might write somewhere in memory.
+ * This isn't true for all calls, but it's safe to tell
+ * gcc that it might happen so it doesn't get clever. */
: "memory");
return call;
}
+/*:*/
void async_hcall(unsigned long call,
unsigned long arg1, unsigned long arg2, unsigned long arg3);
@@ -52,31 +72,40 @@
u32 eax, edx, ebx, ecx;
};
-/* All the good stuff happens here: guest registers it with LGUEST_INIT */
+/*G:032 The second method of communicating with the Host is to via "struct
+ * lguest_data". The Guest's very first hypercall is to tell the Host where
+ * this is, and then the Guest and Host both publish information in it. :*/
struct lguest_data
{
-/* Fields which change during running: */
- /* 512 == enabled (same as eflags) */
+ /* 512 == enabled (same as eflags in normal hardware). The Guest
+ * changes interrupts so often that a hypercall is too slow. */
unsigned int irq_enabled;
- /* Interrupts blocked by guest. */
+ /* Fine-grained interrupt disabling by the Guest */
DECLARE_BITMAP(blocked_interrupts, LGUEST_IRQS);
- /* Virtual address of page fault. */
+ /* The Host writes the virtual address of the last page fault here,
+ * which saves the Guest a hypercall. CR2 is the native register where
+ * this address would normally be found. */
unsigned long cr2;
- /* Async hypercall ring. 0xFF == done, 0 == pending. */
+ /* Async hypercall ring. Instead of directly making hypercalls, we can
+ * place them in here for processing the next time the Host wants.
+ * This batching can be quite efficient. */
+
+ /* 0xFF == done (set by Host), 0 == pending (set by Guest). */
u8 hcall_status[LHCALL_RING_SIZE];
+ /* The actual registers for the hypercalls. */
struct hcall_ring hcalls[LHCALL_RING_SIZE];
-/* Fields initialized by the hypervisor at boot: */
+/* Fields initialized by the Host at boot: */
/* Memory not to try to access */
unsigned long reserve_mem;
- /* ID of this guest (used by network driver to set ethernet address) */
+ /* ID of this Guest (used by network driver to set ethernet address) */
u16 guestid;
/* KHz for the TSC clock. */
u32 tsc_khz;
-/* Fields initialized by the guest at boot: */
+/* Fields initialized by the Guest at boot: */
/* Instruction range to suppress interrupts even if enabled */
unsigned long noirq_start, noirq_end;
};