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: <7b30f2b5-5173-4c3b-85ff-dbfeda3c807a@linux.intel.com>
Date: Thu, 6 Nov 2025 09:55:39 +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 11/5/2025 10:43 PM, Sean Christopherson wrote:
> 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.
>
I can send a patch.