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

On Fri, 24 Aug 2018 08:11:48 -0300 Eduardo Habkost <ehabkost(a)redhat.com&gt; wrote: > On Fri, Aug 24, 2018 at 11:13:50AM +0200, Igor Mammedov wrote: > > On Thu, 23 Aug 2018 18:32:41 +0200 > > Paolo Bonzini <pbonzini(a)redhat.com&gt; 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) > > This destination CLI aren't exactly correct as well since > > it should've been exactly the same -smp as on source + a bunch of -device cpufoo... > > so we can always say go fix your CLI so it won't trigger error. > > > > > The destination should have sockets=8, shouldn't it? > > either that or cores=8 or cores=4,sockets=2 ... > > > > > 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. > > I think that s*t*c should describe topology of whole machine > > including not yet plugged vcpus. "cpus = s*t*c" probably won't work > > for partially filled package case: > > -smp 1,cores=1,threads=8,sockets=1 > > cores/threads should reflect full package configuration > > for guest to see an expected topology. > > Oh, now I remember: that's the reason we don't enforce > s*t*c == smp_cpus nor s*t*c == max_cpus. > > Both "-smp 4,maxcpus=8,cores=2,threads=2,sockets=1" and > "-smp 4,maxcpus=8,cores=2,threads=2,sockets=2" > worked since maxcpus was introduced, making the semantics of > "sockets" unclear and hard to change without breaking existing > configs. Should we go with deprication thingy then, so we could make it clear in the future?