Re: [PATCH v2 3/3] kvm: svm: Use the hardware provided GPA instead of page walk

Brijesh Singh
2016-12-08T17:14:05Z

Hi Paolo,

On 12/08/2016 08:52 AM, Paolo Bonzini wrote:
>
>
> On 23/11/2016 18:02, Brijesh Singh wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> 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@amd.com>
>> Reviewed-by: Borislav Petkov <bp@suse.de>
>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>
> Tom, Brijesh,
>
> I would like to confirm that you have run kvm-unit-tests with this patch?
> I haven't yet tried AMD (will do before sending the pull request to Linus),
> but a similar patch for Intel gives me these 4 failures on emulator.flat:
>
> FAIL: push mem
> FAIL: pop mem
> FAIL: cross-page mmio read
> FAIL: cross-page mmio write
>

I did not ran kvm-unit-tests. However, I was able to boot Linux
guest and run some stress test. I will run kvm-unit-tests and let
you know soon.

-Brijesh
> The VMX patch to set gpa_available is just this:
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 25d48380c312..5d7b60d4795b 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6393,6 +6393,7 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
> /* ept page table is present? */
> error_code |= (exit_qualification & 0x38) != 0;
>
> + vcpu->arch.gpa_available = true;
> vcpu->arch.exit_qualification = exit_qualification;
>
> return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
> @@ -6410,6 +6411,7 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
> }
>
> ret = handle_mmio_page_fault(vcpu, gpa, true);
> + vcpu->arch.gpa_available = true;
> if (likely(ret == RET_MMIO_PF_EMULATE))
> return x86_emulate_instruction(vcpu, gpa, 0, NULL, 0) ==
> EMULATE_DONE;
> @@ -8524,6 +8526,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
> u32 vectoring_info = vmx->idt_vectoring_info;
>
> trace_kvm_exit(exit_reason, vcpu, KVM_ISA_VMX);
> + vcpu->arch.gpa_available = false;
>
> /*
> * Flush logged GPAs PML buffer, this will make dirty_bitmap more
>
> Thanks,
>
> Paolo
>
>> ---
>> arch/x86/include/asm/kvm_emulate.h | 3 +++
>> arch/x86/include/asm/kvm_host.h | 3 +++
>> arch/x86/kvm/svm.c | 2 ++
>> arch/x86/kvm/x86.c | 17 ++++++++++++++++-
>> 4 files changed, 24 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
>> index e9cd7be..2d1ac09 100644
>> --- a/arch/x86/include/asm/kvm_emulate.h
>> +++ b/arch/x86/include/asm/kvm_emulate.h
>> @@ -344,6 +344,9 @@ struct x86_emulate_ctxt {
>> struct read_cache mem_read;
>> };
>>
>> +/* String operation identifier (matches the definition in emulate.c) */
>> +#define CTXT_STRING_OP (1 << 13)
>> +
>> /* Repeat String Operation Prefix */
>> #define REPE_PREFIX 0xf3
>> #define REPNE_PREFIX 0xf2
>> 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/svm.c b/arch/x86/kvm/svm.c
>> index 5e64e656..1bbd04c 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -4246,6 +4246,8 @@ static int handle_exit(struct kvm_vcpu *vcpu)
>> return 1;
>> }
>>
>> + vcpu->arch.gpa_available = (exit_code == SVM_EXIT_NPF);
>> +
>> return svm_exit_handlers[exit_code](svm);
>> }
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index c30f62dc..5002eea 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4441,7 +4441,19 @@ static int vcpu_mmio_gva_to_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
>> return 1;
>> }
>>
>> - *gpa = vcpu->arch.walk_mmu->gva_to_gpa(vcpu, gva, access, exception);
>> + /*
>> + * 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 when the NPF occurred.
>> + */
>> + if (vcpu->arch.gpa_available &&
>> + !(vcpu->arch.emulate_ctxt.d & CTXT_STRING_OP))
>> + *gpa = exception->address;
>> + else
>> + *gpa = vcpu->arch.walk_mmu->gva_to_gpa(vcpu, gva, access,
>> + exception);
>>
>> if (*gpa == UNMAPPED_GVA)
>> return -1;
>> @@ -5563,6 +5575,9 @@ 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)
>>
>> --
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