Re: [PATCH] KVM: x86: Advertise that the SMM_CTL MSR is not supported

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 07, 2022, Jim Mattson wrote:
> CPUID.80000021H:EAX[bit 9] indicates that the SMM_CTL MSR (0xc0010116)
> is not supported. This defeature can be advertised by
> KVM_GET_SUPPORTED_CPUID regardless of whether or not the host
> enumerates it.

Might be worth noting that KVM will only enumerate the bit if the host happens to
have a max extend leaf > 80000021.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>  arch/x86/kvm/cpuid.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 2796dde06302..b748fac2ae37 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1199,8 +1199,12 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		 * Other defined bits are for MSRs that KVM does not expose:
>  		 *   EAX      3      SPCL, SMM page configuration lock
>  		 *   EAX      13     PCMSR, Prefetch control MSR
> +		 *
> +		 * KVM doesn't support SMM_CTL.
> +		 *   EAX       9     SMM_CTL MSR is not supported
>  		 */
>  		entry->eax &= BIT(0) | BIT(2) | BIT(6);

I don't suppose I can bribe you to add a kvm_only_cpuid_leafs entry for these? :-)

> +		entry->eax |= BIT(9);
>  		if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC))
>  			entry->eax |= BIT(2);
>  		if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
> -- 
> 2.38.0.rc1.362.ged0d419d3c-goog
>