[libvirt] [PATCH v2 04/23] qemu: Forbid config when topology based cpu count doesn't match the config

[Peter Krempa]

On Fri, Aug 19, 2016 at 13:10:09 -0400, John Ferlan wrote:
> 
> 
> On 08/19/2016 10:38 AM, Peter Krempa wrote:
> > As of qemu commit:
> > commit a32ef3bfc12c8d0588f43f74dcc5280885bbdb30
> > Author: Thomas Huth <thuth at redhat.com>
> > Date:   Wed Jul 22 15:59:50 2015 +0200
> > 
> >     vl: Add another sanity check to smp_parse() function
> > 
> > v2.4.0-952-ga32ef3b
> > 
> > configuration where the maximum CPU count doesn't match the topology is
> > rejected. Prior to that only configurations where the topology would
> > contain more cpus than the maximum count would be rejected.
> > 
> > Use QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS as a relevant recent enough
> > witness to avoid breaking old configs.
> > ---
> > 
> > Notes:
> >     v2:
> >     -fixed typo in commit message
> >     - already ACKed
> > 
> >  src/qemu/qemu_domain.c | 38 ++++++++++++++++++++++++++++++++------
> >  1 file changed, 32 insertions(+), 6 deletions(-)

[...]

> > +    /* qemu as of 2.5.0 rejects SMP topologies that don't match the cpu count */
> > +    if (def->cpu && def->cpu->sockets) {
> > +        topologycpus = def->cpu->sockets * def->cpu->cores * def->cpu->threads;
> > +        if (topologycpus != virDomainDefGetVcpusMax(def)) {
> > +            /* presence of query-hotpluggable-cpus should be a good enough witness */
> > +            if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS)) {
> > +                virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
> > +                               _("CPU topology doesn't match maximum vcpu count"));
> > +                goto cleanup;
> > +            }
> 
> Would it be a good idea to add a VIR_WARN or VIR_INFO (something) that
> will give a heads up that the configuration may prevent some future
> start? Especially since you went through all the math before detecting
> the capability...

Well, adding a warning would be cerainly easy, but as it was discussed
earlier, using VIR_WARN or VIR_INFO doesn't make much sense since it's
improbable that the user will be reading logs after defining a VM.

Peter