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

[Sean Christopherson <seanjc@google.com>]

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@amd.com> wrote:
> >>
> >> On 12/7/20 5:22 PM, Jim Mattson wrote:
> >>> On Mon, Dec 7, 2020 at 2:38 PM Babu Moger <babu.moger@amd.com> 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.