Re: KVM PMU virtualization

[Jes Sorensen <Jes.Sorensen@redhat.com>]

On 02/26/10 12:24, Ingo Molnar wrote:
> There is a way to query the CPU for 'architectural perfmon' though, via CPUID
> alone - that is how we set the X86_FEATURE_ARCH_PERFMON shortcut. The logic
> is:
>
>          if (c->cpuid_level>  9) {
>                  unsigned eax = cpuid_eax(10);
>                  /* Check for version and the number of counters */
>                  if ((eax&  0xff)&&  (((eax>>8)&  0xff)>  1))
>                          set_cpu_cap(c, X86_FEATURE_ARCH_PERFMON);
>          }
>
> But emulating that doesnt solve the problem: as OSs generally dont key their
> PMU drivers off the relatively new 'architectural perfmon' CPUID detail, but
> based on much higher level CPUID attributes. (like Intel/AMD)

Right, there is far more to it than just the arch-perfmon feature. They
still need to query cpuid 0x0a for counter size, number of counters and
stuff like that.

Cheers,
Jes