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

[Sean Christopherson <seanjc@google.com>]

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@google.com/
> > 
> 
> 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.

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.