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Re: [Qemu-devel] [PATCH v3 07/10] check: Only test boot-serial when sga

From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] [PATCH v3 07/10] check: Only test boot-serial when sga is compiled in
Date: Thu, 23 Aug 2018 10:53:59 +0100
User-agent: Mutt/1.10.1 (2018-07-13)

On Thu, Aug 23, 2018 at 11:48:50AM +0200, Gerd Hoffmann wrote:
> > > I remember we even discussed that a while back ...
> > > Oh yes: 
> > > https://git.kraxel.org/cgit/qemu/commit/?h=work/no-sgabios&id=408690999429fdf44011c49b3af6efa50f369b1e
> > > Somehow never ended up being merged.
> > 
> > Sounds like a good idea! Could you repost the patch, with an update to
> > qemu-deprecated.texi, please? Maybe also print the deprecation message
> > in any case if !use_sgabios, e.g.:
> > 
> >     } else {
> >         /*
> >          * For this machine type we expect SeaBIOS to provide a
> >          * serial console for -machine graphics=off.  No need to
> >          * add the sgabios rom here.
> >          */
> >         warn_report("sgabios is deprecated and is not required anymore,"
> >                     " SeaBIOS now has the same functionality, too");
> >     }
> Hmm, IIRC libvirt support is a bit tricky here as there is no easy way
> to probe for serial console support in seabios.  So the idea was to just
> allow both -machine graphics=off and -device sga, by turning -device sga
> into a nop in case -machine graphics=off is present.
> But given we meanwhile had some releases without this I suspect that
> isn't a good idea any more.  With both -machine graphics=off -device sga
> seabios and sgabios will fight over the serial device ...
> So, maybe take a completely different route.  Add a deprecation_reason
> field to DeviceClass.  Make it visible in qapi introspection, so libvirt
> can see it and avoid adding it to the command line.  Then we can
> deprecate -device sga (and possibly other devices too) that way.

That won't work IIUC. If we stop using "-device sga" based on some
introspected deprecation value on the device, that means the guest
will have a different ABI from POV of migration stream. So we would
be unable to migrate to a QEMU without the deprecation, despite the
XML config being unchanged.

The only way we can stop using "-device sga" is if we either have
something explicit in the XML saying whether to use it or not, or
alternatively have some indicator associated with the next versioned
machine type that we can hook off.

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