On 19/01/2016 17:54, Michael S. Tsirkin wrote: > On Fri, Jan 15, 2016 at 01:41:57PM +0100, Paolo Bonzini wrote: >> From: Vincenzo Maffione <v.maffi...@gmail.com> >> >> The virtqueue_pop() implementation needs to check if the avail ring >> contains some pending buffers. To perform this check, it is not >> always necessary to fetch the avail_idx in the VQ memory, which is >> expensive. This patch introduces a shadow variable tracking avail_idx >> and modifies virtio_queue_empty() to access avail_idx in physical >> memory only when necessary. >> >> Signed-off-by: Vincenzo Maffione <v.maffi...@gmail.com> >> Message-Id: >> <b617d6459902773d9f4ab843bfaca764f5af8eda.1450218353.git.v.maffi...@gmail.com> >> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> > > Is the cost due to the page walk?
Yes, as with all the other patches. But unlike patches 7 and 10 where we just reduce the number of walks, for patch 8 and 9 it's difficult to beat a local cache. :) >> @@ -1579,6 +1595,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int >> version_id) >> return -1; >> } >> vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]); >> + vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]); >> } >> } > > > shadow_avail_idx also should be updated on vhost stop, That's virtio_queue_set_last_avail_idx, right? Paolo
