[libvirt] vcpupin reports bogus vcpu affinities

[Cole Robinson]

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