Re: [Qemu-devel] [PATCH v3] virtio-pci: Fix cross-version migration with older machines

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Dec 15, 2016 at 09:26:56AM +0100, Cornelia Huck wrote:
> On Thu, 15 Dec 2016 07:38:09 +0000
> Stefan Hajnoczi <stefanha@gmail.com> wrote:
> 
> > On Wed, Dec 14, 2016 at 05:30:35PM +0100, Maxime Coquelin wrote:
> > > This patch fixes a cross-version migration regression introduced
> > > by commit d1b4259f ("virtio-bus: Plug devices after features are
> > > negotiated").
> > > 
> > > The problem is encountered when host's vhost backend does not support
> > > VIRTIO_F_VERSION_1, and migration is initiated from a v2.7 or prior
> > > machine with virtio-pci modern capabilities enabled to a v2.8 machine.
> > > 
> > > In this case, modern capabilities get exposed to the guest by the source,
> > > whereas the target will detect version 1 is not supported so will only
> > > expose legacy capabilities.
> > > 
> > > The problem is fixed by introducing a new "x-ignore-backend-features"
> > > property, which is set in v2.7 and prior compatibility modes. Doing this,
> > > v2.7 machine keeps its broken behaviour (enabling modern while version
> > > is not supported), and newer machines will behave correctly.
> > > 
> > > Reported-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> > > Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> > > Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
> > > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> > > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > > ---
> > > 
> > > V3 fixes commit message with the new property name.
> > > V2 changes the naming as proposed by Michael T.and Cornelia, and
> > > fixes commit message.
> > > 
> > >  hw/virtio/virtio-pci.c | 5 ++++-
> > >  hw/virtio/virtio-pci.h | 1 +
> > >  include/hw/compat.h    | 4 ++++
> > >  3 files changed, 9 insertions(+), 1 deletion(-)
> > 
> > Thanks, applied to my staging tree:
> > https://github.com/stefanha/qemu/commits/staging
> > 
> > Stefan
> 
> Hm, missing my R-b I gave for v2... feel free to add
> 
> Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> 
> if you still want to change it.

Sorry, the R-b didn't make it into v2.8.0-rc4.

The patch management scripts I use apply all R-b for a single email
thread.  They do not look at previous revisions (it would be hard to
tell which R-b still stand and which do not).  The usual solution is for
the patch author to include previous R-b in new revisions as long as
code changes aren't substantial.

Stefan

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