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

[Brijesh Singh]

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)
-- 
2.7.4