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

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 24 Aug 2018 10:53:32 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Fri, Aug 24, 2018 at 01:26:54PM +0200, Igor Mammedov wrote:
> > On Fri, 24 Aug 2018 08:11:48 -0300
> > Eduardo Habkost <ehabkost@redhat.com> 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@redhat.com> 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?  
> 
> Yes, but I'm not sure which option we should adopt
> (s*t*c == smp_cpus or s*t*c == max_cpus).
s*t*c == smp_cpus is wrong as one won't be able to start QEMU
with partial package and then hotplug the rest when needed.
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