virtio: introduce avail cache

This allows skipping avail ring writes when they
don't need to change. Good for cache locality.

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

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 096b857e7b75abad526f487c84392d863c3c6b06..14e7ce9b3e96b1abf0239b801a7cc0f1d682c01a 100644 (file)
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -91,6 +91,7 @@ struct vring_virtqueue {
        bool last_add_time_valid;
        ktime_t last_add_time;
 #endif
+       u16 *avail;
 
        /* Tokens for callbacks. */
        void *data[];
@@ -236,7 +237,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
        /* Put entry in available array (but don't update avail->idx until they
         * do sync). */
        avail = virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) & (vq->vring.num - 1);
-       vq->vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
+       if (vq->avail[avail] != head) {
+               vq->avail[avail] = head;
+               vq->vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
+       }
 
        /* Descriptors and available array need to be set before we expose the
         * new available array entries. */
@@ -724,6 +728,11 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
        vq = kmalloc(sizeof(*vq) + sizeof(void *)*num, GFP_KERNEL);
        if (!vq)
                return NULL;
+       vq->avail = kzalloc(sizeof (*vq->avail) * num, GFP_KERNEL);
+       if (!vq->avail) {
+               kfree(vq);
+               return NULL;
+       }
 
        vring_init(&vq->vring, num, pages, vring_align);
        vq->vq.callback = callback;