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

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(a)redhat.com&gt; 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.

> @@ -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.