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Re: [libvirt] [PATCH] qemu: ensure "pc" machine is always used as default if available



On Fri, Aug 03, 2018 at 01:05:49PM -0300, Eduardo Habkost wrote:
> On Fri, Aug 03, 2018 at 01:59:47PM +0100, Daniel P. Berrangé wrote:
> > It is increasingly likely that some distro is going to change the
> > default "x86" machine type in QEMU from "pc" to "q35". This will
> > certainly break existing applications which write their XML on the
> > assumption that its using a "pc" machine by default. For example they'll
> > lack a IDE CDROM and get PCI-X instad of PCI which changes the topology
> > radically.
> > 
> > Libvirt promises to isolate applications from hypervisor changes that
> > may cause incompatibilities, so we must ensure that we always use the
> > "pc" machine type if it is available. Only use QEMU's own reported
> > default machine type if "pc" does not exist.
> > 
> > Note this change assumes there will always be a "pc" alias as long as a
> > versioned "pc-XXX" machine type exists. If QEMU were to ship a "pc-XXX"
> > machine type but not provide the "pc" alias, it is too hard to decide
> > which to default so. Versioned machine types are supposed to be
> > considered opaque strings, so we can't apply any sensible ordering
> > ourselves and QEMU isn't reporting the list of machines in any sensible
> > ordering itself.
> > 
> > Signed-off-by: Daniel P. Berrangé <berrange redhat com>
> 
> Won't this break qemuParseCommandLine() if it sees a QEMU binary
> running without "-machine"?  It will assume the QEMU default is
> "pc" but this may be not true.

If no -machine arg is present in ARGV, then the code will lookup the
default machine type for the emulator binary in the capabilities
record. So this should just "do the right thing" with my changes
in this patch.

> > @@ -2263,12 +2274,16 @@ virQEMUCapsProbeQMPMachineTypes(virQEMUCapsPtr qemuCaps,
> >          mach->maxCpus = machines[i]->maxCpus;
> >          mach->hotplugCpus = machines[i]->hotplugCpus;
> >  
> > +        if (STREQ_NULLABLE(mach->alias, preferredAlias))
> > +            preferredIdx = qemuCaps->nmachineTypes - 1;
> > +
> 
> Isn't STREQ_NULLABLE(NULL, NULL) true?  You don't want to set
> preferredIdx here if preferredAlias==NULL.

Opps, yes.

Regards,
Daniel
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