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

[Wei Wang]

On 01/08/2019 02:48 AM, 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.

Probably one more check would be better:

if (!boot_cpu_has(X86_FEATURE_PDCM) ||
    !guest_cpuid_has(vcpu, X86_FEATURE_PDCM))
    return 1;

(host isn't expected to read this MSR when PDCM is not supported
by the guest, so don't have "!msr_info->host_initiate" added to above)

Best,
Wei