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

[Babu Moger]

On 12/9/20 5:11 PM, Jim Mattson wrote:
> 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?
> 

Working on v2 of these patches. Saw this code comment(in
arch/x86/kvm/cpuid.c) on about exposing SVM features to the guest.


        /*
         * Hide all SVM features by default, SVM will set the cap bits for
         * features it emulates and/or exposes for L1.
         */
        kvm_cpu_cap_mask(CPUID_8000_000A_EDX, 0);


Should we go ahead with the changes here?

Thanks
Babu