Re: [Qemu-devel] [PATCH] vl.c: make sure maxcpus matches topology to prevent migration failure

[Eduardo Habkost]

On Thu, Aug 23, 2018 at 06:32:41PM +0200, Paolo Bonzini wrote:
> On 23/08/2018 16:51, Igor Mammedov wrote:
> > Topology (threads*cores*sockets) must match maxcpus to be valid,
> > otherwise we could start QEMU with invalid topology that throws
> > a error on migration destination side, that should not be reachable:
> > Source:
> >   -smp 8,maxcpus=64,cores=1,threads=8,sockets=1
> > // hotplug cpus upto maxcpus
> > Destination:
> >   -smp 64,maxcpus=64,cores=1,threads=8,sockets=1
> >   qemu: cpu topology: sockets (1) * cores (1) * threads (8) < smp_cpus (64)
> 
> The destination should have sockets=8, shouldn't it?
> 
> It seems to me that, at startup, you should have cpus = s*t*c and cpus
> <= maxcpus.  Currently we check cpus <= s*t*c <= maxcpus, which doesn't
> make much sense.

Most of the incompleteness of input validation at smp_parse() can
be explained by our fear of breaking existing configurations and
making existing running VMs not runnable.

But now we have a deprecation policy.  If we're still afraid of
breaking peoples' existing configurations, we should at least
deprecate those configurations as soon as possible (and make QEMU
at least emit a warning).

-- 
Eduardo