linux-kernel - Re: [PATCH 1/1] x86/kvm/svm: Implement support for PSFD

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Date:   Wed, 7 Apr 2021 20:03:08 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ramakrishna Saripalli <rsaripal@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] x86/kvm/svm: Implement support for PSFD

On Wed, Apr 07, 2021, Ramakrishna Saripalli wrote:
> From: Ramakrishna Saripalli <rk.saripalli@....com>
> 
> Expose Predictive Store Forwarding capability to guests.

Technically KVM is advertising the capability to userspace, e.g. userspace can
expose the feature to the guest without this patch.

> Guests enable or disable PSF via SPEC_CTRL MSR.

At a (very) quick glance, this requires extra enabling in guest_has_spec_ctrl_msr(),
otherwise a vCPU with PSF but not the existing features will not be able to set
MSR_IA32_SPEC_CTRL.PSFD.

That raises a question: should KVM do extra checks for PSFD on top of the "throw
noodles at the wall and see what sticks" approach of kvm_spec_ctrl_test_value()?
The noodle approach is there to handle the mess of cross-vendor features/bits,
but that doesn't seem to apply to PSFD.

> Signed-off-by: Ramakrishna Saripalli <rk.saripalli@....com>
> ---
>  arch/x86/kvm/cpuid.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6bd2f8b830e4..9c4af0fef6d7 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -448,6 +448,8 @@ void kvm_set_cpu_caps(void)
>  		kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
>  	if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
>  		kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
> +	if (boot_cpu_has(X86_FEATURE_AMD_PSFD))
> +		kvm_cpu_cap_set(X86_FEATURE_AMD_PSFD);

This is unnecessary, it's handled by the F(AMD_PSFD).  The above features have
special handling to enumerate their Intel equivalent.

>  	kvm_cpu_cap_mask(CPUID_7_1_EAX,
>  		F(AVX_VNNI) | F(AVX512_BF16)
> @@ -482,7 +484,7 @@ void kvm_set_cpu_caps(void)
>  	kvm_cpu_cap_mask(CPUID_8000_0008_EBX,
>  		F(CLZERO) | F(XSAVEERPTR) |
>  		F(WBNOINVD) | F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | F(VIRT_SSBD) |
> -		F(AMD_SSB_NO) | F(AMD_STIBP) | F(AMD_STIBP_ALWAYS_ON)
> +		F(AMD_SSB_NO) | F(AMD_STIBP) | F(AMD_STIBP_ALWAYS_ON) | F(AMD_PSFD)
>  	);
>  
>  	/*
> -- 
> 2.25.1
>