Re: [PATCH v10 3/9] KVM: x86: Use KVM-governed feature framework to track "LAM enabled"

[Huang, Kai]

On Wed, 2023-07-19 at 22:41 +0800, Binbin Wu wrote:
> Use the governed feature framework to track if Linear Address Masking (LAM)
> is "enabled", i.e. if LAM can be used by the guest. So that guest_can_use()
> can be used to support LAM virtualization.

Better to explain why to use governed feature for LAM?  Is it because there's
hot path(s) calling guest_cpuid_has()?  Anyway some context of why can help
here.

> 
> LAM modifies the checking that is applied to 64-bit linear addresses, allowing
> software to use of the untranslated address bits for metadata and masks the
> metadata bits before using them as linear addresses to access memory.
> 
> Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/governed_features.h | 2 ++
>  arch/x86/kvm/vmx/vmx.c           | 3 +++
>  2 files changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/governed_features.h b/arch/x86/kvm/governed_features.h
> index 40ce8e6608cd..708578d60e6f 100644
> --- a/arch/x86/kvm/governed_features.h
> +++ b/arch/x86/kvm/governed_features.h
> @@ -5,5 +5,7 @@ BUILD_BUG()
>  
>  #define KVM_GOVERNED_X86_FEATURE(x) KVM_GOVERNED_FEATURE(X86_FEATURE_##x)
>  
> +KVM_GOVERNED_X86_FEATURE(LAM)
> +
>  #undef KVM_GOVERNED_X86_FEATURE
>  #undef KVM_GOVERNED_FEATURE
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 0ecf4be2c6af..ae47303c88d7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7783,6 +7783,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  		vmx->msr_ia32_feature_control_valid_bits &=
>  			~FEAT_CTL_SGX_LC_ENABLED;
>  
> +	if (boot_cpu_has(X86_FEATURE_LAM))
> +		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LAM);
> +

If you want to use boot_cpu_has(), it's better to be done at your last patch to
only set the cap bit when boot_cpu_has() is true, I suppose.

>  	/* Refresh #PF interception to account for MAXPHYADDR changes. */
>  	vmx_update_exception_bitmap(vcpu);
>  }