linux-kernel - Re: [PATCH 1/2] x86/cpufeatures: Add the Virtual SPEC

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Date:   Wed, 9 Dec 2020 15:11:15 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Babu Moger <babu.moger@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        kvm list <kvm@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        kyung.min.park@...el.com, LKML <linux-kernel@...r.kernel.org>,
        Krish Sadhukhan <krish.sadhukhan@...cle.com>,
        "H . Peter Anvin" <hpa@...or.com>, mgross@...ux.intel.com,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kim.phillips@....com,
        wei.huang2@....com
Subject: Re: [PATCH 1/2] x86/cpufeatures: Add the Virtual SPEC_CTRL feature

On Wed, Dec 9, 2020 at 2:39 PM Babu Moger <babu.moger@....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@....com> 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@....com>
> >> ---
> >>  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?