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

From: Sean Christopherson <seanjc@google.com>
To: Ramakrishna Saripalli <rsaripal@amd.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] x86/kvm/svm: Implement support for PSFD
Date: Wed, 7 Apr 2021 20:03:08 +0000	[thread overview]
Message-ID: <YG4P/EdPN4qGpYUq@google.com> (raw)
In-Reply-To: <20210407194512.6922-1-rsaripal@amd.com>

On Wed, Apr 07, 2021, Ramakrishna Saripalli wrote:
> From: Ramakrishna Saripalli <rk.saripalli@amd.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@amd.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
> 