Re: [PATCH v3] kvm: svm: Use the hardware provided GPA instead of page walk

[Paolo Bonzini]

On 29/11/2016 19:01, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
> When a guest causes a NPF which requires emulation, KVM sometimes walks
> the guest page tables to translate the GVA to a GPA. This is unnecessary
> most of the time on AMD hardware since the hardware provides the GPA in
> EXITINFO2.
> 
> The only exception cases involve string operations involving rep or
> operations that use two memory locations. With rep, the GPA will only be
> the value of the initial NPF and with dual memory locations we won't know
> which memory address was translated into EXITINFO2.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Reviewed-by: Borislav Petkov <bp@xxxxxxx>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/include/asm/kvm_emulate.h |    1 +
>  arch/x86/include/asm/kvm_host.h    |    3 ++
>  arch/x86/kvm/emulate.c             |    8 +++++++
>  arch/x86/kvm/svm.c                 |    2 ++
>  arch/x86/kvm/x86.c                 |   44 ++++++++++++++++++++++++++++--------
>  5 files changed, 48 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index e9cd7be..777eea2 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -441,5 +441,6 @@ int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
>  int emulate_int_real(struct x86_emulate_ctxt *ctxt, int irq);
>  void emulator_invalidate_register_cache(struct x86_emulate_ctxt *ctxt);
>  void emulator_writeback_register_cache(struct x86_emulate_ctxt *ctxt);
> +bool emulator_is_string_op(struct x86_emulate_ctxt *ctxt);
>  
>  #endif /* _ASM_X86_KVM_X86_EMULATE_H */
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 77cb3f9..fd5b1c8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -668,6 +668,9 @@ struct kvm_vcpu_arch {
>  
>  	int pending_ioapic_eoi;
>  	int pending_external_vector;
> +
> +	/* GPA available (AMD only) */
> +	bool gpa_available;
>  };
>  
>  struct kvm_lpage_info {
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index a3ce9d2..0ea543e 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -5483,3 +5483,11 @@ void emulator_writeback_register_cache(struct x86_emulate_ctxt *ctxt)
>  {
>  	writeback_registers(ctxt);
>  }
> +
> +bool emulator_is_string_op(struct x86_emulate_ctxt *ctxt)
> +{
> +	if (ctxt->d & String)
> +		return true;
> +
> +	return false;
> +}
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 5e64e656..e9b3555 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4188,6 +4188,8 @@ static int handle_exit(struct kvm_vcpu *vcpu)
>  
>  	trace_kvm_exit(exit_code, vcpu, KVM_ISA_SVM);
>  
> +	vcpu->arch.gpa_available = (exit_code == SVM_EXIT_NPF);
> +
>  	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
> index c30f62dc..507c75c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4420,6 +4420,21 @@ int kvm_write_guest_virt_system(struct x86_emulate_ctxt *ctxt,
>  }
>  EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
>  
> +static int vcpu_is_mmio_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
> +			    gpa_t gpa, bool write)
> +{
> +	/* For APIC access vmexit */
> +	if ((gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
> +		return 1;
> +
> +	if (vcpu_match_mmio_gpa(vcpu, gpa)) {
> +		trace_vcpu_match_mmio(gva, gpa, write, true);
> +		return 1;
> +	}
> +
> +	return 0;
> +}
> +
>  static int vcpu_mmio_gva_to_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
>  				gpa_t *gpa, struct x86_exception *exception,
>  				bool write)
> @@ -4446,16 +4461,7 @@ static int vcpu_mmio_gva_to_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
>  	if (*gpa == UNMAPPED_GVA)
>  		return -1;
>  
> -	/* For APIC access vmexit */
> -	if ((*gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
> -		return 1;
> -
> -	if (vcpu_match_mmio_gpa(vcpu, *gpa)) {
> -		trace_vcpu_match_mmio(gva, *gpa, write, true);
> -		return 1;
> -	}
> -
> -	return 0;
> +	return vcpu_is_mmio_gpa(vcpu, gva, *gpa, write);
>  }
>  
>  int emulator_write_phys(struct kvm_vcpu *vcpu, gpa_t gpa,
> @@ -4552,6 +4558,21 @@ static int emulator_read_write_onepage(unsigned long addr, void *val,
>  	int handled, ret;
>  	bool write = ops->write;
>  	struct kvm_mmio_fragment *frag;
> +	struct x86_emulate_ctxt *ctxt = &vcpu->arch.emulate_ctxt;
> +
> +	/*
> +	 * If the exit was due to a NPF we may already have a GPA.
> +	 * If the GPA is present, use it to avoid the GVA to GPA table walk.
> +	 * Note, this cannot be used on string operations since string
> +	 * operation using rep will only have the initial GPA from the NPF
> +	 * occurred.
> +	 */
> +	if (vcpu->arch.gpa_available &&
> +	    !emulator_is_string_op(ctxt) &&
> +	    vcpu_is_mmio_gpa(vcpu, addr, exception->address, write)) {
> +		gpa = exception->address;
> +		goto mmio;
> +	}
>  
>  	ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write);
>  
> @@ -5563,6 +5584,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>  	}
>  
>  restart:
> +	/* Save the faulting GPA (cr2) in the address field */
> +	ctxt->exception.address = cr2;
> +
>  	r = x86_emulate_insn(ctxt);
>  
>  	if (r == EMULATION_INTERCEPTED)
> 

Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Thanks!

Paolo