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

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Message-ID: <CALMp9eTk6B2832EN8EhL51m8UqmHLTfeOjdKs8TvFSSAUxGk2Q@mail.gmail.com>
Date:   Mon, 7 Dec 2020 15:22:00 -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 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?

>  /* Intel-defined CPU features, CPUID level 0x00000007:0 (ECX), word 16 */
>  #define X86_FEATURE_AVX512VBMI         (16*32+ 1) /* AVX512 Vector Bit Manipulation instructions*/
>