On Mon, 2011-03-14 at 22:30 +0200, Michael S. Tsirkin wrote:
> Generally, the Host end of the virtio ring doesn't need to see where
> Guest is up to in consuming the ring. However, to completely
> understand
> what's going on from the outside, this information must be exposed.
> For example, host can reduce the number of interrupts by detecting
> that the guest is currently handling previous buffers.
>
> Fortunately, we have room to expand: the ring is always a whole number
> of pages and there's hundreds of bytes of padding after the avail ring
> and the used ring, whatever the number of descriptors (which must be a
> power of 2).
>
> We add a feature bit so the guest can tell the host that it's writing
> out the current value there, if it wants to use that.
>
> This is based on a patch by Rusty Russell, with the main difference
> being that we dedicate a feature bit to guest to tell the host it is
> writing the used index. This way we don't need to force host to
> publish
> the last available index until we have a use for it.
>
> Memory barriers use has been rationalized (hopefully correctly).
> To avoid the cost of an extra memory barrier on each update, we only
> commit for the index to be up to date when interrupts
> are enabled.
>
> Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> drivers/virtio/virtio_ring.c | 25 ++++++++++++++++---------
> include/linux/virtio_ring.h | 11 +++++++++++
> 2 files changed, 27 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c
> b/drivers/virtio/virtio_ring.c
> index cc2f73e..a6fc537 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -89,9 +89,6 @@ struct vring_virtqueue
> /* Number we've added since last sync. */
> unsigned int num_added;
>
> - /* Last used index we've seen. */
> - u16 last_used_idx;
> -
> /* How to notify other side. FIXME: commonalize hcalls! */
> void (*notify)(struct virtqueue *vq);
>
> @@ -283,12 +280,13 @@ static void detach_buf(struct vring_virtqueue
> *vq, unsigned int head)
>
> static inline bool more_used(const struct vring_virtqueue *vq)
> {
> - return vq->last_used_idx != vq->vring.used->idx;
> + return vring_last_used(vq) != vq->vring.used->idx;
> }
here should be vring_last_used(&vq->vring)
> void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> + struct vring_used_elem *u;
> void *ret;
> unsigned int i;
>
> @@ -308,9 +306,9 @@ void *virtqueue_get_buf(struct virtqueue *_vq,
> unsigned int *len)
> /* Only get used array entries after they have been exposed by
> host. */
> virtio_rmb();
>
> - i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
> - *len = vq->vring.used->ring[vq->last_used_idx%
> vq->vring.num].len;
> -
> + u = &vq->vring.used->ring[vring_last_used(vq) %
> vq->vring.num];
same here vring_last_used(&vq->vring)
> + i = u->id;
> + *len = u->len;
> if (unlikely(i >= vq->vring.num)) {
> BAD_RING(vq, "id %u out of range\n", i);
> return NULL;
> @@ -323,7 +321,12 @@ void *virtqueue_get_buf(struct virtqueue *_vq,
> unsigned int *len)
> /* detach_buf clears data, so grab it now. */
> ret = vq->data[i];
> detach_buf(vq, i);
> - vq->last_used_idx++;
> + (vring_last_used(vq))++;
> + /* If we expect an interrupt for the next entry, flush out
> + * last used index write. */
> + if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
> + virtio_mb();
> +
> END_USE(vq);
> return ret;
> }
> @@ -346,6 +349,8 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
> /* We optimistically turn back on interrupts, then check if
> there was
> * more to do. */
> vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> + /* Besides flags write, this barrier also flushes out
> + * last available index write. */
> virtio_mb();
> if (unlikely(more_used(vq))) {
> END_USE(vq);
> @@ -429,7 +434,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int
> num,
> vq->vq.name = name;
> vq->notify = notify;
> vq->broken = false;
> - vq->last_used_idx = 0;
> + vring_last_used(vq) = 0;
Same here vring_last_used(&vq->vring)
> vq->num_added = 0;
> list_add_tail(&vq->vq.list, &vdev->vqs);
> #ifdef DEBUG
> @@ -471,6 +476,8 @@ void vring_transport_features(struct virtio_device
> *vdev)
> switch (i) {
> case VIRTIO_RING_F_INDIRECT_DESC:
> break;
> + case VIRTIO_RING_F_PUBLISH_USED:
> + break;
> default:
> /* We don't understand this bit. */
> clear_bit(i, vdev->features);
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index e4d144b..4587ea2 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -29,6 +29,9 @@
> /* We support indirect buffer descriptors */
> #define VIRTIO_RING_F_INDIRECT_DESC 28
>
> +/* The Guest publishes last-seen used index at the end of the avail
> ring. */
> +#define VIRTIO_RING_F_PUBLISH_USED 29
> +
> /* Virtio ring descriptors: 16 bytes. These can chain together via
> "next". */
> struct vring_desc {
> /* Address (guest-physical). */
> @@ -83,6 +86,7 @@ struct vring {
> * __u16 avail_flags;
> * __u16 avail_idx;
> * __u16 available[num];
> + * __u16 last_used_idx;
> *
> * // Padding to the next align boundary.
> * char pad[];
> @@ -93,6 +97,10 @@ struct vring {
> * struct vring_used_elem used[num];
> * };
> */
> +
> +/* We publish the last-seen used index at the end of the available
> ring.
> + * It is at the end for backwards compatibility. */
> +#define vring_last_used(vr) ((vr)->avail->ring[num])
> static inline void vring_init(struct vring *vr, unsigned int num,
> void *p,
> unsigned long align)
> {
> @@ -101,6 +109,9 @@ static inline void vring_init(struct vring *vr,
> unsigned int num, void *p,
> vr->avail = p + num*sizeof(struct vring_desc);
> vr->used = (void *)(((unsigned long)&vr->avail->ring[num] +
> align-1)
> & ~(align - 1));
> + /* Verify that last used index does not spill over the used
> ring. */
> + BUG_ON((void *)&vring_last_used(vr) +
> + sizeof vring_last_used(vr) > (void *)vr->used);
> }
>
> static inline unsigned vring_size(unsigned int num, unsigned long
> align)
> --
> 1.7.3.2.91.g446ac
>
>
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