Comment # 16 on bug 1202541 from Bernhard Wiedemann

Another interesting find:

atreju6-suse:~ # cat /proc/cpuinfo |grep lwp|wc -l
     16
atreju6-suse:~ # cat /proc/cpuinfo |grep nodeid_msr|wc -l
     32

Only half of the cores have the lwp bit set and restarting libvirtd can make it
appear or disappear in the virsh capabilities feature flags - probably if the
process gets scheduled on the right 50%.

http://developer.amd.com/wordpress/media/2012/10/43724.pdf p23 says
> LWP is supported on a processor if CPUID Fn8000_0001_ECX[LWP] (bit 15) is set.
> This bit is identical to the value of CPUID Fn0000_000D_EDX_x0[bit 30],
> which is bit 62 of the XFeatureSupportedMask and indicates XSAVE support
> for LWP. A system can check either of those bits to determine if LWP is supported.


With the cpu host-passthrough mode we currently use, this random lwp
availability could be the cause of trouble... unless guests never use it. And
so far I did not find a single VM that listed the bit.