[libvirt] Problem setting CPU topology

[Eduardo Habkost]

On Tue, Jul 10, 2012 at 11:54:05AM +0200, Christophe Fergeau wrote:
> On Sat, Jul 07, 2012 at 07:10:53PM +0300, Zeeshan Ali (Khattak) wrote:
> > Hi,
> >    I'm trying to set exact CPU topology to qemu-kvm domains to match
> > host's topology. In my case, host topology is: 1 socket, 2 cores and 2
> > threads. If I set the XML like this:
> > 
> > <domain type='kvm'>
> >   ..
> >   <vcpu placement='static'>4</vcpu>
> >   <os>
> >     <type arch='i686' machine='pc-0.15'>hvm</type>
> >     <boot dev='hd'/>
> >   </os>
> >   <cpu mode='host-model'>
> >     <model fallback='allow'/>
> >     <topology sockets='1' cores='2' threads='2'/>
> >   </cpu>
> > ..
> > 
> > The qemu commandline launched for this domain looks like this:
> > 
> > /usr/bin/qemu-kvm -name fedora17-2 -S -M pc-0.15 -cpu
> > core2duo,+lahf_lm,+rdtscp,+aes,+popcnt,+sse4.2,+sse4.1,+pdcm,+xtpr,+cx16,+tm2,+est,+smx,+vmx,+ds_cpl,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds
> > -enable-kvm -m 1152 -smp 4,sockets=1,cores=2,threads=2 -uuid
> > c573342b-2876-05b8-098e-6d5314cab062 -nodefconfig -nodefaults -chardev
> > socket,id=charmonitor,path=/home/zeenix/.config/libvirt/qemu/lib/fedora17-2.monitor,server,nowait
> > -mon chardev=charmonitor,id=monitor,mode=control -rtc
> > base=utc,driftfix=slew -no-kvm-pit-reinjection -no-shutdown -no-acpi
> 
> I debugged this together with Zeeshan, and the issue comes from this -no-acpi flag
> which libvirt added because the domain XML was missing
> <features><acpi/></features>
> So in the end that was a bug in Boxes, not really a libvirt/qemu issue.
> Unless libvirt/qemu should be taught that CPU topology won't work without ACPI
> and complain about it?

I don't think it's really impossible for any guest OS to recognize the
CPU topology or boot the other CPUs without ACPI. It looks like it's
just a limitation of (most?) guest OSes. If that's the case, libvirt or
QEMU can't prevent the user from running a (possibly valid)
configuration just because it's not very common.

-- 
Eduardo