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: <88404ae2-fa4b-4357-918b-fd949dd2521a@linux.intel.com>
Date: Wed, 5 Nov 2025 18:42:04 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.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 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);
         }
}