On 23/11/2016 18:02, Brijesh Singh wrote:
> From: Tom Lendacky <[email protected]>
>
> 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 <[email protected]>
> Reviewed-by: Borislav Petkov <[email protected]>
> Signed-off-by: Brijesh Singh <[email protected]>
> ---
> 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)
I'm not sure how I missed this in v1, so this is my fault, but it is
wrong in many ways:
- it doesn't say *what* it matches (it matches "String")
- it doesn't include a check that it actually does match the definition
- it doesn't say with what field you're supposed to use it.
- it is not clear from the code that vcpu->arch.emulate_ctxt is valid at
all in vcpu_mmio_gva_to_gpa.
So please make a non-static function in emulate.c that takes the
emulation context pointer and returns a bool.
Also, in fact, now that I look at it, this path is relatively unlikely
to ever trigger, necause vcpu_match_mmio_gva probably matches first
anyway. But it _is_ cheaper, since it doesn't need to test
permission_fault, so it is indeed a worthwhile patch to have.
However, please place it straight in emulator_read_write_onepage, like
if (vcpu->arch.gpa_available &&
... test CTXT_STRING_OP ... &&
vcpu_is_mmio_gpa(vcpu, exception->address)) {
gpa = exception->address;
goto mmio;
}
where vcpu_is_mmio_gpa is essentially the APIC_DEFAULT_PHYS_BASE +
vcpu_match_mmio_gpa pair of checks, extracted from vcpu_mmio_gva_to_gpa.
Thanks, and sorry for the bad earlier review.
Paolo
> /* 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)
>
