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

[Michael S. Tsirkin]

On Wed, Nov 25, 2015 at 09:32:15AM +0800, Yuanhan Liu wrote:
> On Tue, Nov 24, 2015 at 11:23:34PM +0200, Michael S. Tsirkin wrote:
> > 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 <address@hidden> 
> > > 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 <address@hidden>
> > > >> ---
> > > >>  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.
> 
> Yes, that's what I suggested first, which also makes the logic quite
> simple: we use GET_VRING_BASE as the sign of vring stop. Intead of
> GET_VRING_BASE when protocol not negotiated, and SET_VRING_ENABLE
> when protocol is negotiated.
> 
> Michael, should I submit a revert patch, or you could do it directly?
> 
>       --yliu

I can handle it.

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