Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to the guest

[Andi Kleen <ak@linux.intel.com>]

On Mon, Jan 07, 2019 at 10:48:38AM -0800, Jim Mattson wrote:
> On Mon, Jan 7, 2019 at 10:20 AM Andi Kleen <ak@linux.intel.com> wrote:
> >
> > > The issue is compatibility. Prior to your change, reading this MSR
> > > from a VM would raise #GP. After your change, it won't. That means
> > > that if you have a VM migrating between hosts with kernel versions
> > > before and after this change, the results will be inconsistent. In the
> >
> > No it will not be. All Linux kernel uses of this MSR are guarded
> > by a CPUID check.
> 
> Linux usage is irrelevant to the architected behavior of the virtual
> CPU. According to volume 4 of the SDM, this MSR is only supported when
> CPUID.01H:ECX.PDCM [bit 15] is set. Therefore, kvm should raise #GP
> whenever a guest tries to read this MSR and the guest's
> CPUID.01H:ECX.PDCM [bit 15] is clear.

That's not general practice in KVM. There are lots of MSRs
that don't fault even if their CPUID doesn't support them.

It's also not generally true for instructions, where it is even
impossible.

You could argue it should be done for MSRs, but that would
be a much larger patch for lots of MSRs. It seems pointless
to single out this particular one.

In practice I doubt it will make any difference for
real software either way.

-Andi