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Date: Tue, 11 May 2010 22:48:48 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Ryan Harper <ryanh@...ibm.com>
Cc: Rusty Russell <rusty@...tcorp.com.au>,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
qemu-devel@...gnu.org
Subject: Re: [PATCH RFC] virtio: put last seen used index into ring itself
On Tue, May 11, 2010 at 01:46:08PM -0500, Ryan Harper wrote:
> * Michael S. Tsirkin <mst@...hat.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@...tcorp.com.au>
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.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@...ibm.com
Thanks, this is PUBLISHED_USED all over.
This is fixed in the version I'm testing.
--
MST
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