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

[Tom Lendacky]

On 4/7/21 2:45 PM, Ramakrishna Saripalli wrote:
> From: Ramakrishna Saripalli 
> 
> Expose Predictive Store Forwarding capability to guests.
> Guests enable or disable PSF via SPEC_CTRL MSR.
> 
> Signed-off-by: Ramakrishna Saripalli 
> ---
>  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);
>  
>   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)

Please note that this patch has a pre-req against the PSFD support that
defines this feature:

https://lore.kernel.org/lkml/20210406155004.230790-2-rsari...@amd.com/#t

Thanks,
Tom

>   );
>  
>   /*
> 

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

[Sean Christopherson]

On Wed, Apr 07, 2021, Ramakrishna Saripalli wrote:
> From: Ramakrishna Saripalli 
> 
> 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 
> ---
>  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
> 

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

[Ramakrishna Saripalli]

From: Ramakrishna Saripalli 

Expose Predictive Store Forwarding capability to guests.
Guests enable or disable PSF via SPEC_CTRL MSR.

Signed-off-by: Ramakrishna Saripalli 
---
 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);
 
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