Re: [PATCH 1/2] x86/cpufeatures: Add the Virtual SPEC_CTRL feature

[Jim Mattson]

On Wed, Dec 9, 2020 at 2:39 PM Babu Moger <babu.moger@xxxxxxx> wrote:
>
>
>
> On 12/7/20 5:22 PM, Jim Mattson wrote:
> > On Mon, Dec 7, 2020 at 2:38 PM Babu Moger <babu.moger@xxxxxxx> wrote:
> >>
> >> Newer AMD processors have a feature to virtualize the use of the SPEC_CTRL
> >> MSR. This feature is identified via CPUID 0x8000000A_EDX[20]. When present,
> >> the SPEC_CTRL MSR is automatically virtualized and no longer requires
> >> hypervisor intervention.
> >>
> >> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> >> ---
> >>  arch/x86/include/asm/cpufeatures.h |    1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> >> index dad350d42ecf..d649ac5ed7c7 100644
> >> --- a/arch/x86/include/asm/cpufeatures.h
> >> +++ b/arch/x86/include/asm/cpufeatures.h
> >> @@ -335,6 +335,7 @@
> >>  #define X86_FEATURE_AVIC               (15*32+13) /* Virtual Interrupt Controller */
> >>  #define X86_FEATURE_V_VMSAVE_VMLOAD    (15*32+15) /* Virtual VMSAVE VMLOAD */
> >>  #define X86_FEATURE_VGIF               (15*32+16) /* Virtual GIF */
> >> +#define X86_FEATURE_V_SPEC_CTRL                (15*32+20) /* Virtual SPEC_CTRL */
> >
> > Shouldn't this bit be reported by KVM_GET_SUPPORTED_CPUID when it's
> > enumerated on the host?
>
> Jim, I am not sure if this needs to be reported by
> KVM_GET_SUPPORTED_CPUID. I dont see V_VMSAVE_VMLOAD or VGIF being reported
> via KVM_GET_SUPPORTED_CPUID. Do you see the need for that?

Every little bit helps. No, it isn't *needed*. But then again, this
entire patchset isn't *needed*, is it?