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

[Binbin Wu]

On 8/17/2023 5:33 AM, Sean Christopherson wrote:
> On Wed, Aug 16, 2023, Kai Huang wrote:
> > > > > --- 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/
> > I mean kvm_cpu_cap_has() checks against the host CPUID directly while here you
> > are using boot_cpu_has().  They are not the same.
> >
> > If LAM should be only supported when boot_cpu_has() is true then it seems you
> > can just only set the LAM cap bit when boot_cpu_has() is true.  As you also
> > mentioned above the kvm_governed_feature_check_and_set() here internally does
> > kvm_cpu_cap_has().
> That's covered by the last patch:
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index e961e9a05847..06061c11d74d 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -677,7 +677,7 @@ void kvm_set_cpu_caps(void)
>          kvm_cpu_cap_mask(CPUID_7_1_EAX,
>                  F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD) |
>                  F(FZRM) | F(FSRS) | F(FSRC) |
> -               F(AMX_FP16) | F(AVX_IFMA)
> +               F(AMX_FP16) | F(AVX_IFMA) | F(LAM)
>          );
>   
>          kvm_cpu_cap_init_kvm_defined(CPUID_7_1_EDX,
>
>
> Which highlights a problem with activating a goverened feature before said feature
> is actually supported by KVM: it's all kinds of confusing.
>
> It'll generate a more churn in git history, but I think we should first enable
> LAM without a goverened feature, and then activate a goverened feature later on.
> Using a goverened feature is purely an optimization, i.e. the series needs to be
> function without using a governed feature.
OK, then how about the second option which has been listed in your v9 
patch series discussion.
https://lore.kernel.org/kvm/20230606091842.13123-1-binbin.wu@xxxxxxxxxxxxxxx/T/#m16ee5cec4a46954f985cb6afedb5f5a3435373a1

Temporarily add a bool can_use_lam in kvm_vcpu_arch and use the bool
"can_use_lam" instead of guest_can_use(vcpu, X86_FEATURE_LAM).
and then put the patch of adopting "KVM-governed feature framework" to 
the last.


> That should yield an easier-to-review series on all fronts: the initial supports
> won't have any more hidden dependencies than absolutely necessary, switching to
> a goverened feature should be a very mechanical conversion (if it's not, that's
> a red flag), and last but not least, it makes it super easy to make a judgment
> call as to whether using a governed feature flag is justified, because all of the
> users will be in scope.
>
> TL;DR: Do the whole goverened feature thing dead last.