[Qemu-devel] Re: [PATCH RFC] virtio: put last seen used index into ring itself

[Ryan Harper]

* Michael S. Tsirkin <address@hidden> [2010-05-05 16:37]:
> 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.
> 
> Signed-off-by: Rusty Russell <address@hidden>
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
> 
> Rusty,
> this is a simplified form of a patch you posted in the past.
> I have a vhost patch that, using this feature, shows external
> to host bandwidth grow from 5 to 7 GB/s, by avoiding
> an interrupt in the window after previous interrupt
> was sent and before interrupts were disabled for the vq.
> With vhost under some external to host loads I see
> this window being hit about 30% sometimes.
> 
> I'm finalizing the host bits and plan to send
> the final version for inclusion when all's ready,
> but I'd like to hear comments meanwhile.
> 
>  drivers/virtio/virtio_ring.c |   28 +++++++++++++++++-----------
>  include/linux/virtio_ring.h  |   14 +++++++++++++-
>  2 files changed, 30 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 1ca8890..7729aba 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);
> 
> @@ -285,12 +282,13 @@ 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 *vq->vring.last_used_idx != vq->vring.used->idx;
>  }
> 
>  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;
> 
> @@ -307,12 +305,13 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned 
> int *len)
>               return NULL;
>       }
> 
> -     /* 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;
> +     /* Only get used array entries after they have been exposed by host.
> +      * Need mb(), not just rmb() because we write last_used_idx below. */
> +     virtio_mb();
> 
> +     u = &vq->vring.used->ring[*vq->vring.last_used_idx % vq->vring.num];
> +     i = u->id;
> +     *len = u->len;
>       if (unlikely(i >= vq->vring.num)) {
>               BAD_RING(vq, "id %u out of range\n", i);
>               return NULL;
> @@ -325,7 +324,8 @@ 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++;
> +     (*vq->vring.last_used_idx)++;
> +
>       END_USE(vq);
>       return ret;
>  }
> @@ -431,7 +431,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
>       vq->vq.name = name;
>       vq->notify = notify;
>       vq->broken = false;
> -     vq->last_used_idx = 0;
> +     *vq->vring.last_used_idx = 0;
>       vq->num_added = 0;
>       list_add_tail(&vq->vq.list, &vdev->vqs);
>  #ifdef DEBUG
> @@ -440,6 +440,10 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
> 
>       vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
> 
> +     /* We publish used index whether Host offers it or not: if not, it's
> +      * junk space anyway.  But calling this acknowledges the feature. */
> +     virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_USED);
> +

You use VIRTIO_RING_F_PUBLISH_USED here, but
VIRTIO_RING_F_PUBLISH_INDICES below... 


>       /* No callback?  Tell other side not to bother us. */
>       if (!callback)
>               vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
> @@ -473,6 +477,8 @@ void vring_transport_features(struct virtio_device *vdev)
>               switch (i) {
>               case VIRTIO_RING_F_INDIRECT_DESC:
>                       break;
> +             case VIRTIO_RING_F_PUBLISH_INDICES:
> +                     break;

Here ^^^

>               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..9d01de9 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
> +

And here; is it PUBLISHED_USED or PUBLISHED_INDICIES ?


-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
address@hidden