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

From: Jim Mattson
Date: Mon Jan 07 2019 - 16:00:20 EST

On Mon, Jan 7, 2019 at 12:14 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxx> 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.

KVM doesn't really have a "general practice" in this regard. True,
there are lots of MSRs that don't fault even if unsupported. But there
are quite a few that do fault if unsupported. On the Intel side,
MSR_TSC_AUX all check for the corresponding capabilities in guest
CPUID. In the "common" code, MSR_IA32_TSC_ADJUST,
MSR_AMD64_OSVW_ID_LENGTH, and MSR_AMD64_OSVW_STATUS all check for the
corresponding capabilities in guest CPUID.

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

Yes, sometimes it is impossible to #UD for instructions that are
supported on the host but not in the guest. However, sometimes it is
possible, and kvm sometimes does enforce this. Some examples that come
to mind are RDTSCP and INVPCID. In fact, kvm goes so far as to require
that the guest CPUID must enumerate PCID support in order to enumerate
INVPCID support, and INVPCID will raise #UD unless both capabilities
are enumerated!

> 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.

Past sloppiness isn't really a compelling argument for continued
sloppiness going forward. And there are existing MSRs that do the
appropriate checks, so I'm not singling this one out.

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

Perhaps, but when the correct behavior is so easy to implement, why
not just do it right in the first place?