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Re: [libvirt] [PATCH for-4.0 v2] virtio: Provide version-specific variants of virtio PCI devices



On Mon, 19 Nov 2018 22:44:54 -0200
Eduardo Habkost <ehabkost redhat com> wrote:

> On Thu, Nov 15, 2018 at 11:50:56AM +0100, Cornelia Huck wrote:
> > On Thu, 15 Nov 2018 10:05:59 +0000
> > Daniel P. Berrangé <berrange redhat com> wrote:

> > > If libvirt did this compatibility approach, can you
> > > confirm this would be live migration state compatible.
> > > 
> > > ie can live migrate virtio-*-pci -> virtio-*-pci-transitional,
> > > provided only PCI bus was used.  
> > 
> > It also needs to make sure that neither disable-legacy nor
> > disable-modern is set. Then this would have a compatible state AFAICS.
> >   
> > >   
> > > > - virtio-*-pci-non-transitional: modern-only
> > > >   - Supports both Conventional PCI and PCI Express buses    
> > > 
> > > IIUC, libvirt can again provide compatibility with old
> > > QEMU by simply using the existing device type and setting
> > > disable-legacy ?  Can you confirm this would be live
> > > migration compatible
> > > 
> > >   virtio-*-pci + disable-legacy -> virtio-*pci-non-transitional  
> > 
> > I think yes.  
> 
> This is exactly how it is implemented internally, but I'm not
> promising that this will be kept compatible forever.  And I
> wouldn't like to make that promise unless there's an important
> use case for that.

Shouldn't we be able to ensure compatibility by normal virtio feature
bit handling, as we have already done in the past?

> 
> We could easily ensure it will be compatible in pc-4.0 and older,
> though.  Would that be enough for the use case we have in mind?
> 



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