[PATCH 6/6] virtio: core support for config generation

[Michael S. Tsirkin]

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 <m...@redhat.com>
---
 include/linux/virtio_config.h | 32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index a61cd37..ca3ed78 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);
 }
 
-- 
MST

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