Re: [libvirt] vcpupin reports bogus vcpu affinities

On 3/26/19 4:06 PM, Allen, John wrote: > For pinned vcpus, vcpupin will report inaccurate affinity values on machines > with high core counts (256 cores in my case). The problem is produced as > follows: > > $ virsh vcpupin myguest 0 4 > > $ virsh vcpupin myguest 0 > > VCPU CPU Affinity > --------------------------- > 0 4,192,194,196-197 > > Running taskset on the qemu threads shows the correct affinity, so this seems > to be a reporting problem. Strangely, the value "192" is significant. If I pin > a cpu greater than 192, the problem no longer appears. > > I believe the cause of the problem in my case is that in this case in > src/conf/domain_conf.c:virDomainDefGetVcpuPinInfoHelper: > > ... > if (vcpu && vcpu->cpumask) > bitmap = vcpu->cpumask; > ... > > vcpu->cpumask is "shortened" in that it is only long enough to contain the last > set bit in the mask. However, when we go to copy the mask to the buffer that is > returned, we use the masklen passed to the function which is the "full" > masklen with a bit for each cpu. So it seems virBitmapToDataBuf copies some > extra data past the end of the bitmask. Why the "192" value is always set and I > typically see similar bogus bits set is still unknown. > > What is the function meant to assume in this case? Is it sane to assume that > the bitmask is the full length of the buffer here and it's the responsibility > of the setter of vcpu->cpumask to provide the length of the bitmap we're > expecting? Or should we assume that we may receive a shortened bitmask here and > expand the bitmask before copying to the buffer? >

I didn't dig into the code much but I can try and help figure it out. Can you also provide: * libvirt version

* host distro

* output of 'virsh nodeinfo'

* output of 'virsh nodecpumap'

Thanks, Cole