linux-kernel - Re: [RFC PATCH 26/35] KVM: SVM: Guest FPU state save/restore not needed for SEV-ES guest

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Date:   Tue, 15 Sep 2020 09:25:18 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [RFC PATCH 26/35] KVM: SVM: Guest FPU state save/restore not
 needed for SEV-ES guest

On 9/14/20 4:39 PM, Sean Christopherson wrote:
> On Mon, Sep 14, 2020 at 03:15:40PM -0500, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@....com>
>>
>> The guest FPU is automatically restored on VMRUN and saved on VMEXIT by
>> the hardware, so there is no reason to do this in KVM.
> 
> I assume hardware has its own buffer?  If so, a better approach would be to
> not allocate arch.guest_fpu in the first place, and then rework KVM to key
> off !guest_fpu.

Yup, let me look into that.

Thanks,
Tom

> 
>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>> ---
>>  arch/x86/kvm/svm/svm.c |  8 ++++++--
>>  arch/x86/kvm/x86.c     | 18 ++++++++++++++----
>>  2 files changed, 20 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index b35c2de1130c..48699c41b62a 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -3682,7 +3682,9 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>>  		svm_set_dr6(svm, DR6_FIXED_1 | DR6_RTM);
>>  
>>  	clgi();
>> -	kvm_load_guest_xsave_state(vcpu);
>> +
>> +	if (!sev_es_guest(svm->vcpu.kvm))
>> +		kvm_load_guest_xsave_state(vcpu);
>>  
>>  	if (lapic_in_kernel(vcpu) &&
>>  		vcpu->arch.apic->lapic_timer.timer_advance_ns)
>> @@ -3728,7 +3730,9 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>>  	if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
>>  		kvm_before_interrupt(&svm->vcpu);
>>  
>> -	kvm_load_host_xsave_state(vcpu);
>> +	if (!sev_es_guest(svm->vcpu.kvm))
>> +		kvm_load_host_xsave_state(vcpu);
>> +
>>  	stgi();
>>  
>>  	/* Any pending NMI will happen here */
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 76efe70cd635..a53e24c1c5d1 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -8896,9 +8896,14 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
>>  
>>  	kvm_save_current_fpu(vcpu->arch.user_fpu);
>>  
>> -	/* PKRU is separately restored in kvm_x86_ops.run.  */
>> -	__copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,
>> -				~XFEATURE_MASK_PKRU);
>> +	/*
>> +	 * An encrypted save area means that the guest state can't be
>> +	 * set by the hypervisor, so skip trying to set it.
>> +	 */
>> +	if (!vcpu->arch.vmsa_encrypted)
>> +		/* PKRU is separately restored in kvm_x86_ops.run. */
>> +		__copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,
>> +					~XFEATURE_MASK_PKRU);
>>  
>>  	fpregs_mark_activate();
>>  	fpregs_unlock();
>> @@ -8911,7 +8916,12 @@ static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>>  {
>>  	fpregs_lock();
>>  
>> -	kvm_save_current_fpu(vcpu->arch.guest_fpu);
>> +	/*
>> +	 * An encrypted save area means that the guest state can't be
>> +	 * read/saved by the hypervisor, so skip trying to save it.
>> +	 */
>> +	if (!vcpu->arch.vmsa_encrypted)
>> +		kvm_save_current_fpu(vcpu->arch.guest_fpu);
>>  
>>  	copy_kernel_to_fpregs(&vcpu->arch.user_fpu->state);
>>  
>> -- 
>> 2.28.0
>>