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SubjectRe: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave_state
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Nice one.  The patch can be made simpler though (I think).

On 07/05/21 18:44, Jon Kohler wrote:
@@ -122,7 +124,7 @@ static inline u32 rdpkru(void)
> return 0;
> }
>
> -static inline void __write_pkru(u32 pkru)
> +static inline void __write_pkru(u32 pkru, bool skip_comparison)
> {
> }
> #endif
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index cebdaa1e3cf5..cd95adbd140c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -912,10 +912,10 @@ void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
> }
>
> if (static_cpu_has(X86_FEATURE_PKU) &&
> - (kvm_read_cr4_bits(vcpu, X86_CR4_PKE) ||
> - (vcpu->arch.xcr0 & XFEATURE_MASK_PKRU)) &&
> - vcpu->arch.pkru != vcpu->arch.host_pkru)
> - __write_pkru(vcpu->arch.pkru);
> + vcpu->arch.pkru != vcpu->arch.host_pkru &&
> + ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
> + kvm_read_cr4_bits(vcpu, X86_CR4_PKE)))
> + __write_pkru(vcpu->arch.pkru, false);

This can be optimized as well, can't it? This means that the only case
that needs the rdpkru is in switch_fpu_finish, and __write_pkru can be
removed completely:

- do the rdpkru+wrpkru in switch_fpu_finish

- just use wrpkru in KVM

Paolo


> }
> EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);
>
> @@ -925,11 +925,11 @@ void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
> return;
>
> if (static_cpu_has(X86_FEATURE_PKU) &&
> - (kvm_read_cr4_bits(vcpu, X86_CR4_PKE) ||
> - (vcpu->arch.xcr0 & XFEATURE_MASK_PKRU))) {
> + ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
> + kvm_read_cr4_bits(vcpu, X86_CR4_PKE))) {
> vcpu->arch.pkru = rdpkru();
> if (vcpu->arch.pkru != vcpu->arch.host_pkru)
> - __write_pkru(vcpu->arch.host_pkru);
> + __write_pkru(vcpu->arch.host_pkru, true);
> }
>
> if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE)) {
> --
> 2.30.1 (Apple Git-130)
>

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