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

[Babu Moger]

On 12/22/20 11:41 AM, Sean Christopherson wrote:
> On Tue, Dec 22, 2020, Babu Moger wrote:
>>
>> 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:
>>>>>> 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?
> 
> Probably not, as the current SVM implementation aligns with the intended use of
> KVM_GET_SUPPORTED_CPUID.  The current approach is to enumerate what SVM features
> KVM can virtualize or emulate for a nested VM, i.e. what SVM features an L1 VMM
> can use and thus can be set in a vCPU's CPUID model.  For V_SPEC_CTRL, I'm
> pretty sure Jim was providing feedback for the non-nested case of reporting
> host/KVM support of the feature itself.
> 
> There is the question of whether or not KVM should have an ioctl() to report
> what virtualization features are supported/enabled.  AFAIK, it's not truly
> required as userspace can glean the information via /proc/cpuinfo (especially
> now that vmx_features exists), raw CPUID, and KVM module params.  Providing an
> ioctl() would likely be a bit cleaner for userspace, but I'm guessing that ship
> has already sailed for most VMMs.
> 

Sean, Thanks for the clarifications.