[PATCH] x86: kvm: Avoid guest page table walk when gpa_available is set

Brijesh Singh
2017-04-24T15:54:15Z

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@amd.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;
};

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;
+
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 ccbd45e..18ec746 100644
--- 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;

r = x86_emulate_insn(ctxt);

--
1.9.1