Re: [PATCH v2] KVM: x86: Avoid guest page table walk when gpa_available is set

[Brijesh Singh]

Hi Paolo and Radim

Any comments on this patch, I could not find it in 4.13-2 branch.

Please let me know if you want to fix something, or want me to
refresh and resend the patch.

- Brijesh

On 05/19/2017 10:12 AM, Brijesh Singh wrote:
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>
> On AMD hardware when a guest causes a NPF 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@xxxxxxx>
> ---
> v1: http://marc.info/?l=kvm&m=149304930814202&w=2
>
> Changes in v2:
>   - move gpa_val setting in pf_interception
>
>   arch/x86/include/asm/kvm_host.h |  1 +
>   arch/x86/kvm/svm.c              |  4 ++++
>   arch/x86/kvm/x86.c              | 14 +++++++-------
>   3 files changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 695605e..cc87e00 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -678,6 +678,7 @@ struct kvm_vcpu_arch {
>   
>   	/* GPA available (AMD only) */
>   	bool gpa_available;
> +	gpa_t gpa_val;
>   };
>   
>   struct kvm_lpage_info {
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index c27ac69..27fb563 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2070,9 +2070,13 @@ static void svm_set_dr7(struct kvm_vcpu *vcpu, unsigned long value)
>   static int pf_interception(struct vcpu_svm *svm)
>   {
>   	u64 fault_address = svm->vmcb->control.exit_info_2;
> +	struct kvm_vcpu *vcpu = &svm->vcpu;
>   	u64 error_code;
>   	int r = 1;
>   
> +	/* On #NPF, exit_info_2 contains a valid GPA */
> +	vcpu->arch.gpa_val = fault_address;
> +
>   	switch (svm->apf_reason) {
>   	default:
>   		error_code = svm->vmcb->control.exit_info_1;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b54125b..d2d88ed 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4634,16 +4634,16 @@ static int emulator_read_write_onepage(unsigned long addr, void *val,
>   	 */
>   	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)) {
> -		gpa = exception->address;
> -		goto mmio;
> -	}
> +		gpa = vcpu->arch.gpa_val;
> +		ret = vcpu_is_mmio_gpa(vcpu, addr, gpa, write);
> +	} else {
>   
> -	ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write);
> +		ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write);
>   
> -	if (ret < 0)
> -		return X86EMUL_PROPAGATE_FAULT;
> +		if (ret < 0)
> +			return X86EMUL_PROPAGATE_FAULT;
> +	}
>   
>   	/* For APIC access vmexit */
>   	if (ret)