virtio: explicit advertisement of driver features
A recent proposed feature addition to the virtio block driver revealed
some flaws in the API: in particular, we assume that feature
negotiation is complete once a driver's probe function returns.
There is nothing in the API to require this, however, and even I
didn't notice when it was violated.
So instead, we require the driver to specify what features it supports
in a table, we can then move the feature negotiation into the virtio
core. The intersection of device and driver features are presented in
a new 'features' bitmap in the struct virtio_device.
Note that this highlights the difference between Linux unsigned-long
bitmaps where each unsigned long is in native endian, and a
straight-forward little-endian array of bytes.
Drivers can still remove feature bits in their probe routine if they
really have to.
API changes:
- dev->config->feature() no longer gets and acks a feature.
- drivers should advertise their features in the 'feature_table' field
- use virtio_has_feature() for extra sanity when checking feature bits
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index 475572e..50db245 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -20,11 +20,6 @@
/**
* virtio_config_ops - operations for configuring a virtio device
- * @feature: search for a feature in this config
- * vdev: the virtio_device
- * bit: the feature bit
- * Returns true if the feature is supported. Acknowledges the feature
- * so the host can see it.
* @get: read the value of a configuration field
* vdev: the virtio_device
* offset: the offset of the configuration field
@@ -50,10 +45,15 @@
* callback: the virqtueue callback
* Returns the new virtqueue or ERR_PTR() (eg. -ENOENT).
* @del_vq: free a virtqueue found by find_vq().
+ * @get_features: get the array of feature bits for this device.
+ * vdev: the virtio_device
+ * Returns the first 32 feature bits (all we currently need).
+ * @set_features: confirm what device features we'll be using.
+ * vdev: the virtio_device
+ * feature: the first 32 feature bits
*/
struct virtio_config_ops
{
- bool (*feature)(struct virtio_device *vdev, unsigned bit);
void (*get)(struct virtio_device *vdev, unsigned offset,
void *buf, unsigned len);
void (*set)(struct virtio_device *vdev, unsigned offset,
@@ -65,8 +65,30 @@
unsigned index,
void (*callback)(struct virtqueue *));
void (*del_vq)(struct virtqueue *vq);
+ u32 (*get_features)(struct virtio_device *vdev);
+ void (*set_features)(struct virtio_device *vdev, u32 features);
};
+/* If driver didn't advertise the feature, it will never appear. */
+void virtio_check_driver_offered_feature(const struct virtio_device *vdev,
+ unsigned int fbit);
+
+/**
+ * virtio_has_feature - helper to determine if this device has this feature.
+ * @vdev: the device
+ * @fbit: the feature bit
+ */
+static inline bool virtio_has_feature(const struct virtio_device *vdev,
+ unsigned int fbit)
+{
+ /* Did you forget to fix assumptions on max features? */
+ if (__builtin_constant_p(fbit))
+ BUILD_BUG_ON(fbit >= 32);
+
+ virtio_check_driver_offered_feature(vdev, fbit);
+ return test_bit(fbit, vdev->features);
+}
+
/**
* virtio_config_val - look for a feature and get a virtio config entry.
* @vdev: the virtio device
@@ -84,7 +106,7 @@
unsigned int offset,
void *buf, unsigned len)
{
- if (!vdev->config->feature(vdev, fbit))
+ if (!virtio_has_feature(vdev, fbit))
return -ENOENT;
vdev->config->get(vdev, offset, buf, len);