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

[Ryan Harper <ryanh@us.ibm.com>]

* Michael S. Tsirkin <mst@redhat.com> [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 <rusty@rustcorp.com.au>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> 
> 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
ryanh@us.ibm.com