Re: [libvirt] [Qemu-devel] Problem setting CPU topology

On Tue, Jul 10, 2012 at 2:02 PM, Eduardo Habkost <ehabkost(a)redhat.com&gt; wrote: > 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 > Isn't MPS and APIC support required for a guest OS to really handle the topology correctly? Now days ACPI provides most of that information so its likely that QEMU doesn't support plain old MPS + APIC. Its also possible that QEMU does support this but the kernel got confused because I see the CPUID +acpi specified in the command line while -no-acpi is on the command line as you noted.