* Michael S. Tsirkin <mst@xxxxxxxxxx> [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@xxxxxxxxxxxxxxx> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > --- > > 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@xxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html