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

[Jim Mattson]

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.