linux-kernel - Re: [PATCH] KVM: x86: Use cpu_feature_enabled() for PKU instead of #ifdef

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Date:   Fri, 2 Jun 2023 11:30:21 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Jon Kohler <jon@...anix.com>, Mingwei Zhang <mizhang@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Use cpu_feature_enabled() for PKU instead of #ifdef

On Fri, Jun 2, 2023 at 8:51 AM Jon Kohler <jon@...anix.com> wrote:
>
>
>
> > On Jun 1, 2023, at 9:05 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Replace an #ifdef on CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS with a
> > cpu_feature_enabled() check on X86_FEATURE_PKU.  The macro magic of
> > DISABLED_MASK_BIT_SET() means that cpu_feature_enabled() provides the
> > same end result (no code generated) when PKU is disabled by Kconfig.
> >
> > No functional change intended.
> >
> > Cc: Jon Kohler <jon@...anix.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> > arch/x86/kvm/x86.c | 8 ++------
> > 1 file changed, 2 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ceb7c5e9cf9e..eed1f0629023 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1017,13 +1017,11 @@ void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
> >                       wrmsrl(MSR_IA32_XSS, vcpu->arch.ia32_xss);
> >       }
> >
> > -#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> > -     if (static_cpu_has(X86_FEATURE_PKU) &&
> > +     if (cpu_feature_enabled(X86_FEATURE_PKU) &&
> >           vcpu->arch.pkru != vcpu->arch.host_pkru &&
> >           ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
> >            kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE)))
> >               write_pkru(vcpu->arch.pkru);
> > -#endif /* CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS */
> > }
> > EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);
> >
> > @@ -1032,15 +1030,13 @@ void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
> >       if (vcpu->arch.guest_state_protected)
> >               return;
> >
> > -#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> > -     if (static_cpu_has(X86_FEATURE_PKU) &&
> > +     if (cpu_feature_enabled(X86_FEATURE_PKU) &&
> >           ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
> >            kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE))) {
> >               vcpu->arch.pkru = rdpkru();
> >               if (vcpu->arch.pkru != vcpu->arch.host_pkru)
> >                       write_pkru(vcpu->arch.host_pkru);
> >       }
> > -#endif /* CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS */
> >
> >       if (kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE)) {
> >
> >
> > base-commit: a053a0e4a9f8c52f3acf8a9d2520c4bf39077a7e
> > --
> > 2.41.0.rc2.161.g9c6817b8e7-goog
> >
>
> Thanks for the cleanup!
>
> Reviewed-by: Jon Kohler <jon@...anix.com>

+Mingwei Zhang

As we move towards enabling PKRU on the host, due to some customer
requests, I have to wonder if PKRU-disabled is the norm.

In other words, is this a likely() or unlikely() optimization?