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



On Mon, Aug 06, 2018 at 11:38:06AM +0200, Andrea Bolognani wrote:
> On Fri, 2018-08-03 at 13:59 +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
> 
> s/its/it's/
> 
> > lack a IDE CDROM and get PCI-X instad of PCI which changes the topology
> 
> s/PCI-X instad/PCIe instead/
> 
> [...]
> > +    /* Historically QEMU defaulted to 'pc' machine type but in
> > +     * future might switch 'q35'. Such a change is considered
> > +     * an ABI break from lbivirt's POV, so we must be sure to
> 
> s/lbivirt/libvirt/
> 
> > +     * keep 'pc' as default machine no matter what QEMU says.
> > +     */
> > +    if (qemuCaps->arch == VIR_ARCH_X86_64 ||
> > +        qemuCaps->arch == VIR_ARCH_I686)
> > +        preferredAlias = "pc";
> 
> You can use ARCH_IS_X86() here.
> 
> Since we're shielding users from changes in the default x86
> machine type, I think it would make sense to do the same for other
> architectures at the same time: for example, ppc64 should prefer
> pseries, s390 should prefer s390-ccw-virtio and so on.
> 
> I wonder how to handle architectures where QEMU never declared a
> default machine type, such as aarch64 and riscv64, though: I think
> it would make sense to prefer the virt machine type there, but I'm
> not entirely sure that wouldn't cause any breakages either...

Existing libvirt behaviour is that we'll pick the first reported machine
type, so we have to preserve that.


Regards,
Daniel
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