linux-kernel - Re: [PATCH] x86: kvm: Avoid guest page table walk when gpa

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Date:   Mon, 24 Apr 2017 22:52:36 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     pbonzini@...hat.com, joro@...tes.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, Thomas.Lendacky@....com
Subject: Re: [PATCH] x86: kvm: Avoid guest page table walk when gpa_available
 is set

2017-04-24 11:52-0400, Brijesh Singh:
> On AMD hardware when a guest causes a NFP which requires emulation,
> the vcpu->arch.gpa_available flag is set to indicate that cr2 contains
> a valid GPA.
> 
> Currently, emulator_read_write_onepage() makes use of gpa_available flag
> to avoid a guest page walk for a known MMIO regions. Lets not limit
> the gpa_available optimization to just MMIO region. The patch extends
> the check to avoid page walk whenever gpa_available flag is set.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/svm.c              |  4 ++++
>  arch/x86/kvm/x86.c              | 26 +++++++++++++++-----------
>  3 files changed, 20 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 74ef58c..491326d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -677,6 +677,7 @@ struct kvm_vcpu_arch {
>  
>  	/* GPA available (AMD only) */
>  	bool gpa_available;
> +	gpa_t gpa_val;

Can't we pass this information through function parameters?

(I'd rather avoid intractable variables.)

>  };
>  
>  struct kvm_lpage_info {
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 5fba706..8827e4b 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4159,6 +4159,10 @@ static int handle_exit(struct kvm_vcpu *vcpu)
>  
>  	vcpu->arch.gpa_available = (exit_code == SVM_EXIT_NPF);
>  
> +	/* On #NPF, exit_info_2 contain a valid GPA */
> +	if (vcpu->arch.gpa_available)
> +		vcpu->arch.gpa_val = svm->vmcb->control.exit_info_2;

How is vcpu->arch.gpa_val used between here and the NPF handler?

> +
>  	if (!is_cr_intercept(svm, INTERCEPT_CR0_WRITE))
>  		vcpu->arch.cr0 = svm->vmcb->save.cr0;
>  	if (npt_enabled)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -4653,18 +4653,16 @@ static int emulator_read_write_onepage(unsigned long addr, void *val,
>  	 * occurred.
>  	 */
>  	if (vcpu->arch.gpa_available &&
> -	    emulator_can_use_gpa(ctxt) &&
> -	    vcpu_is_mmio_gpa(vcpu, addr, exception->address, write) &&
> -	    (addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) {
> +		emulator_can_use_gpa(ctxt) &&
> +		(addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) {
>  		gpa = exception->address;
> -		goto mmio;
> +		ret = vcpu_is_mmio_gpa(vcpu, addr, gpa, write);
> +	} else {
> +		ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write);
> +		if (ret < 0)
> +			return X86EMUL_PROPAGATE_FAULT;
>  	}
>  
> -	ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write);
> -
> -	if (ret < 0)
> -		return X86EMUL_PROPAGATE_FAULT;
> -
>  	/* For APIC access vmexit */
>  	if (ret)
>  		goto mmio;
> @@ -5675,8 +5673,14 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>  	}
>  
>  restart:
> -	/* Save the faulting GPA (cr2) in the address field */
> -	ctxt->exception.address = cr2;
> +	/*
> +	 * Save the faulting GPA (cr2) in the address field
> +	 * NOTE: If gpa_available is set then gpa_val will contain a valid GPA
> +	 */
> +	if (vcpu->arch.gpa_available)
> +		ctxt->exception.address = vcpu->arch.gpa_val;
> +	else
> +		ctxt->exception.address = cr2;

And related, shouldn't vcpu->arch.gpa_val be in cr2?

Thanks.