linux-kernel - Re: [PATCH] KVM: nSVM: Optimize L12 to L2 vmcb.save copies

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Message-ID: <9c7e4dec-2181-1720-5981-3ae25c5bb0d9@redhat.com>
Date:   Tue, 2 Mar 2021 07:56:31 -0500
From:   Cathy Avery <cavery@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, wei.huang2@....com
Subject: Re: [PATCH] KVM: nSVM: Optimize L12 to L2 vmcb.save copies

On 3/1/21 7:59 PM, Sean Christopherson wrote:
> On Mon, Mar 01, 2021, Cathy Avery wrote:
>>   	kvm_set_rflags(&svm->vcpu, vmcb12->save.rflags | X86_EFLAGS_FIXED);
>>   	svm_set_efer(&svm->vcpu, vmcb12->save.efer);
>>   	svm_set_cr0(&svm->vcpu, vmcb12->save.cr0);
>>   	svm_set_cr4(&svm->vcpu, vmcb12->save.cr4);
> Why not utilize VMCB_CR?
I was going to tackle CR in a follow up patch. I should have mentioned 
that but it makes sense to go ahead and do it now.
>
>> -	svm->vcpu.arch.cr2 = vmcb12->save.cr2;
>> +	svm->vmcb->save.cr2 = svm->vcpu.arch.cr2 = vmcb12->save.cr2;
> Same question for VMCB_CR2.
>
> Also, isn't writing svm->vmcb->save.cr2 unnecessary since svm_vcpu_run()
> unconditionally writes it?
>
> Alternatively, it shouldn't be too much work to add proper dirty tracking for
> CR2.  VMX has to write the real CR2 every time because there's no VMCS field,
> but I assume can avoid the write and dirty update on the majority of VMRUNs.

I 'll take a look at CR2 as well.

Thanks for the feedback,

Cathy

>
>> +
>>   	kvm_rax_write(&svm->vcpu, vmcb12->save.rax);
>>   	kvm_rsp_write(&svm->vcpu, vmcb12->save.rsp);
>>   	kvm_rip_write(&svm->vcpu, vmcb12->save.rip);
>>   
>>   	/* In case we don't even reach vcpu_run, the fields are not updated */
>> -	svm->vmcb->save.cr2 = svm->vcpu.arch.cr2;
>>   	svm->vmcb->save.rax = vmcb12->save.rax;
>>   	svm->vmcb->save.rsp = vmcb12->save.rsp;
>>   	svm->vmcb->save.rip = vmcb12->save.rip;
>>   
>> -	svm->vmcb->save.dr7 = vmcb12->save.dr7 | DR7_FIXED_1;
>> -	svm->vcpu.arch.dr6  = vmcb12->save.dr6 | DR6_ACTIVE_LOW;
>> -	vmcb_mark_dirty(svm->vmcb, VMCB_DR);
>> +	/* These bits will be set properly on the first execution when new_vmc12 is true */
>> +	if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_DR))) {
>> +		svm->vmcb->save.dr7 = vmcb12->save.dr7 | DR7_FIXED_1;
>> +		svm->vcpu.arch.dr6  = vmcb12->save.dr6 | DR6_ACTIVE_LOW;
>> +		vmcb_mark_dirty(svm->vmcb, VMCB_DR);
>> +	}
>>   }
>>   
>>   static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 54610270f66a..9761a7ca8100 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -1232,6 +1232,7 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>>   	svm->asid = 0;
>>   
>>   	svm->nested.vmcb12_gpa = 0;
>> +	svm->nested.last_vmcb12_gpa = 0;
> We should use INVALID_PAGE, '0' is a legal physical address and could
> theoretically get a false negative on the "new_vmcb12" check.
>
>>   	vcpu->arch.hflags = 0;
>>   
>>   	if (!kvm_pause_in_guest(vcpu->kvm)) {
>> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
>> index fbbb26dd0f73..911868d4584c 100644
>> --- a/arch/x86/kvm/svm/svm.h
>> +++ b/arch/x86/kvm/svm/svm.h
>> @@ -93,6 +93,7 @@ struct svm_nested_state {
>>   	u64 hsave_msr;
>>   	u64 vm_cr_msr;
>>   	u64 vmcb12_gpa;
>> +	u64 last_vmcb12_gpa;
>>   
>>   	/* These are the merged vectors */
>>   	u32 *msrpm;
>> @@ -247,6 +248,11 @@ static inline void vmcb_mark_dirty(struct vmcb *vmcb, int bit)
>>   	vmcb->control.clean &= ~(1 << bit);
>>   }
>>   
>> +static inline bool vmcb_is_dirty(struct vmcb *vmcb, int bit)
>> +{
>> +        return !test_bit(bit, (unsigned long *)&vmcb->control.clean);
>> +}
>> +
>>   static inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
>>   {
>>   	return container_of(vcpu, struct vcpu_svm, vcpu);
>> -- 
>> 2.26.2
>>