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Message-ID: <ZN1Ardu9GRx7KlAV@google.com>
Date: Wed, 16 Aug 2023 14:33:33 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
Chao Gao <chao.gao@...el.com>,
"David.Laight@...LAB.COM" <David.Laight@...lab.com>,
Guang Zeng <guang.zeng@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"pbonzini@...hat.com" <pbonzini@...hat.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"robert.hu@...ux.intel.com" <robert.hu@...ux.intel.com>
Subject: Re: [PATCH v10 3/9] KVM: x86: Use KVM-governed feature framework to
track "LAM enabled"
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.
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