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

[Binbin Wu]

On 8/16/2023 11:46 AM, Huang, Kai wrote:
> 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.
Yes, to avoid calling guest_cpuid_has() in CR3 handling and instruction 
emulation paths.
I will add the context next version.
Thanks!

> > 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.
Yes, but new version of kvm_governed_feature_check_and_set() of 
KVM-governed feature framework will check against kvm_cpu_cap_has() as well.
I will remove the if statement and call 
kvm_governed_feature_check_and_set()  directly.
https://lore.kernel.org/kvm/20230815203653.519297-2-seanjc@xxxxxxxxxx/


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