linux-kernel - [PATCH] x86: kvm: Avoid guest page table walk when gpa

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Date:   Mon, 24 Apr 2017 11:52:26 -0400
From:   Brijesh Singh <brijesh.singh@....com>
To:     <pbonzini@...hat.com>, <rkrcmar@...hat.com>, <joro@...tes.org>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
        <x86@...nel.org>, <Thomas.Lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: [PATCH] x86: kvm: Avoid guest page table walk when gpa_available is set

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@....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