Re: [Qemu-devel] [PATCH for-2.5 1/1] vhost-user: do not send SET_VRING_ENABLE at start

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

On Tue, Nov 24, 2015 at 10:05:27PM +0100, Thibaut Collet wrote:
> On Tue, Nov 24, 2015 at 9:52 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Tue, Nov 24, 2015 at 05:10:36PM +0100, Thibaut Collet wrote:
> >> This patch reverts partially commit 3a12f32229a.
> >>
> >> In case of live migration several queues can be enabled and not only the first
> >> one. So inform backend that only the first queue is enabled is wrong.
> >>
> >> Since commit 7263a0ad7899 backend is already notified of the state of the vring
> >> through the vring attach operation. This function, called during the startup
> >> sequence, provides the correct state of the vring, even in case of live
> >> migration.
> >>
> >> So nothing has to be added to give the vring state to the backend at the startup.
> >>
> >> Signed-off-by: Thibaut Collet <thibaut.collet@6wind.com>
> >> ---
> >>  hw/virtio/vhost.c | 5 -----
> >>  1 file changed, 5 deletions(-)
> >>
> >> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >> index 1794f0d..870cd12 100644
> >> --- a/hw/virtio/vhost.c
> >> +++ b/hw/virtio/vhost.c
> >> @@ -1226,11 +1226,6 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
> >>          }
> >>      }
> >>
> >> -    if (hdev->vhost_ops->vhost_set_vring_enable) {
> >> -        /* only enable first vq pair by default */
> >> -        hdev->vhost_ops->vhost_set_vring_enable(hdev, hdev->vq_index == 0);
> >> -    }
> >> -
> >>      return 0;
> >>  fail_log:
> >>      vhost_log_put(hdev, false);
> >> --
> >> 2.1.4
> >
> > Yes - and I'm beginning to think that maybe we should revert
> > all of 3a12f32229a then, for symmetry.
> >
> 
> Keep the disable vring on the stop can be useful. For example if the
> VM is rebooted all the vring will be disabled and backend will avoid
> to send packet to the VM in this case (I am not sure the virtio ring
> address is always valid during a reboot and writingg data in this
> memory can cause unexpected behaviour in this case).

I think there's still some confusion:
writing memory can still happen even if you disable the ring
since the TX ring is still processed so we write into the used ring.

We call GET_VRING_BASE on stop and that ensures rings are
stopped.


> > Yunnan, Victor - what do you think?
> >
> > --
> > MST