linux-kernel - Re: [PATCH 3/4] KVM: x86: Load guest/host XCR0 and XSS outside of the fastpath run loop

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Message-ID: <aQtiYwBYtsz6Whwz@google.com>
Date: Wed, 5 Nov 2025 06:43:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jon Kohler <jon@...anix.com>
Subject: Re: [PATCH 3/4] KVM: x86: Load guest/host XCR0 and XSS outside of the
 fastpath run loop

On Wed, Nov 05, 2025, Binbin Wu wrote:
> 
> 
> On 10/31/2025 6:42 AM, Sean Christopherson wrote:
> [...]
> > -void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
> > +static void kvm_load_guest_xfeatures(struct kvm_vcpu *vcpu)
> >   {
> >   	if (vcpu->arch.guest_state_protected)
> >   		return;
> >   	if (kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE)) {
> > -
> >   		if (vcpu->arch.xcr0 != kvm_host.xcr0)
> >   			xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
> > @@ -1217,6 +1216,27 @@ void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
> >   		    vcpu->arch.ia32_xss != kvm_host.xss)
> >   			wrmsrq(MSR_IA32_XSS, vcpu->arch.ia32_xss);
> >   	}
> > +}
> > +
> > +static void kvm_load_host_xfeatures(struct kvm_vcpu *vcpu)
> > +{
> > +	if (vcpu->arch.guest_state_protected)
> > +		return;
> > +
> > +	if (kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE)) {
> > +		if (vcpu->arch.xcr0 != kvm_host.xcr0)
> > +			xsetbv(XCR_XFEATURE_ENABLED_MASK, kvm_host.xcr0);
> > +
> > +		if (guest_cpu_cap_has(vcpu, X86_FEATURE_XSAVES) &&
> > +		    vcpu->arch.ia32_xss != kvm_host.xss)
> > +			wrmsrq(MSR_IA32_XSS, kvm_host.xss);
> > +	}
> > +}
> 
> kvm_load_guest_xfeatures() and kvm_load_host_xfeatures() are almost the same
> except for the guest values VS. host values to set.
> I am wondering if it is worth adding a helper to dedup the code, like:
> 
> static void kvm_load_xfeatures(struct kvm_vcpu *vcpu, u64 xcr0, u64 xss)
> {
>         if (vcpu->arch.guest_state_protected)
>                 return;
> 
>         if (kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE)) {
>                 if (vcpu->arch.xcr0 != kvm_host.xcr0)
>                         xsetbv(XCR_XFEATURE_ENABLED_MASK, xcr0);
> 
>                 if (guest_cpu_cap_has(vcpu, X86_FEATURE_XSAVES) &&
>                     vcpu->arch.ia32_xss != kvm_host.xss)
>                         wrmsrq(MSR_IA32_XSS, xss);
>         }
> }

Nice!  I like it.  Want to send a proper patch (relative to this series)?  Or
I can turn the above into a patch with a Suggested-by.  Either way works for me.