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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 11, 2010 at 01:46:08PM -0500, Ryan Harper wrote:
> * 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


Thanks, this is PUBLISHED_USED all over.
This is fixed in the version I'm testing.

-- 
MST