virtio: core support for config generation

virtio 1.0 spec says:

Drivers MUST NOT assume reads from fields greater than 32 bits wide are
atomic, nor are reads from multiple fields: drivers SHOULD read device
configuration space fields like so:
	u32 before, after;
	do {
		before = get_config_generation(device);
		// read config entry/entries.
		after = get_config_generation(device);
	} while (after != before);

Do exactly this, for transports that support it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 28 insertions, 4 deletions

diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index a61cd37f088c..ca3ed78e5ec7 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -19,6 +19,9 @@
  *      offset: the offset of the configuration field
  *      buf: the buffer to read the field value from.
  *      len: the length of the buffer
+ * @generation: config generation counter
+ *      vdev: the virtio_device
+ *      Returns the config generation counter
  * @get_status: read the status byte
  *      vdev: the virtio_device
  *      Returns the status byte
@@ -60,6 +63,7 @@ struct virtio_config_ops {
                     void *buf, unsigned len);
         void (*set)(struct virtio_device *vdev, unsigned offset,
                     const void *buf, unsigned len);
+        u32 (*generation)(struct virtio_device *vdev);
         u8 (*get_status)(struct virtio_device *vdev);
         void (*set_status)(struct virtio_device *vdev, u8 status);
         void (*reset)(struct virtio_device *vdev);
@@ -301,14 +305,33 @@ static inline u8 virtio_cread8(struct virtio_device *vdev, unsigned int offset)
         return ret;
 }
 
+/* Read @count fields, @bytes each. */
+static inline void __virtio_cread_many(struct virtio_device *vdev,
+                                       unsigned int offset,
+                                       void *buf, size_t count, size_t bytes)
+{
+        u32 old, gen = vdev->config->generation ?
+                vdev->config->generation(vdev) : 0;
+        int i;
+
+        do {
+                old = gen;
+
+                for (i = 0; i < count; i++)
+                        vdev->config->get(vdev, offset + bytes * i,
+                                          buf + i * bytes, bytes);
+
+                gen = vdev->config->generation ?
+                        vdev->config->generation(vdev) : 0;
+        } while (gen != old);
+}
+
+
 static inline void virtio_cread_bytes(struct virtio_device *vdev,
                                       unsigned int offset,
                                       void *buf, size_t len)
 {
-        int i;
-
-        for (i = 0; i < len; i++)
-                vdev->config->get(vdev, offset + i, buf + i, 1);
+        __virtio_cread_many(vdev, offset, buf, len, 1);
 }
 
 static inline void virtio_cwrite8(struct virtio_device *vdev,
@@ -352,6 +375,7 @@ static inline u64 virtio_cread64(struct virtio_device *vdev,
 {
         u64 ret;
         vdev->config->get(vdev, offset, &ret, sizeof(ret));
+        __virtio_cread_many(vdev, offset, &ret, 1, sizeof(ret));
         return virtio64_to_cpu(vdev, (__force __virtio64)ret);
 }