Re: [PATCH v2] x86: KVM: Add feature flag for CPUID.80000021H:EAX[bit 1]

[Borislav Petkov]

On Wed, Oct 04, 2023 at 08:12:37PM -0700, Jim Mattson wrote:
> Define an X86_FEATURE_* flag for CPUID.80000021H:EAX.[bit 1], and
> advertise the feature to userspace via KVM_GET_SUPPORTED_CPUID.
> 
> Per AMD's "Processor Programming Reference (PPR) for AMD Family 19h
> Model 61h, Revision B1 Processors (56713-B1-PUB)," this CPUID bit
> indicates that a WRMSR to MSR_FS_BASE, MSR_GS_BASE, or
> MSR_KERNEL_GS_BASE is non-serializing. This is a change in previously
> architected behavior.
> 
> Effectively, this CPUID bit is a "defeature" bit, or a reverse
> polarity feature bit. When this CPUID bit is clear, the feature
> (serialization on WRMSR to any of these three MSRs) is available. When
> this CPUID bit is set, the feature is not available.
> 
> KVM_GET_SUPPORTED_CPUID must pass this bit through from the underlying
> hardware, if it is set. Leaving the bit clear claims that WRMSR to
> these three MSRs will be serializing in a guest running under
> KVM. That isn't true. Though KVM could emulate the feature by
> intercepting writes to the specified MSRs, it does not do so
> today. The guest is allowed direct read/write access to these MSRs
> without interception, so the innate hardware behavior is preserved
> under KVM.
> 
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> ---
> 
> v1 -> v2: Added justification for this change to the commit message,
>           tweaked the macro name and comment in cpufeatures.h for
> 	  improved clarity.
> 
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  arch/x86/kvm/cpuid.c               | 3 ++-
>  2 files changed, 3 insertions(+), 1 deletion(-)

Reviewed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>

-- 
Regards/Gruss,
    Boris.

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