linux-kernel - [PATCH] KVM: vmx, svm: always run with EFER.NXE=1 when shadow paging is active

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Message-Id: <20191027152323.24326-1-pbonzini@redhat.com>
Date:   Sun, 27 Oct 2019 16:23:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     stable@...r.kernel.org, Joerg Roedel <jroedel@...e.de>
Subject: [PATCH] KVM: vmx, svm: always run with EFER.NXE=1 when shadow paging is active

VMX already does so if the host has SMEP, in order to support the combination of
CR0.WP=1 and CR4.SMEP=1.  However, it is perfectly safe to always do so, and in
fact VMX already ends up running with EFER.NXE=1 on old processors that lack the
"load EFER" controls, because it may help avoiding a slow MSR write.  Removing
all the conditionals simplifies the code.

SVM does not have similar code, but it should since recent AMD processors do
support SMEP.  So this patch also makes the code for the two vendors more similar
while fixing NPT=0, CR0.WP=1 and CR4.SMEP=1 on AMD processors.

Cc: stable@...r.kernel.org
Cc: Joerg Roedel <jroedel@...e.de>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm.c     | 10 ++++++++--
 arch/x86/kvm/vmx/vmx.c | 14 +++-----------
 2 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index b6feb6a11a8d..2c452293c7cc 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -732,8 +732,14 @@ static int get_npt_level(struct kvm_vcpu *vcpu)
 static void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
 {
 	vcpu->arch.efer = efer;
-	if (!npt_enabled && !(efer & EFER_LMA))
-		efer &= ~EFER_LME;
+
+	if (!npt_enabled) {
+		/* Shadow paging assumes NX to be available.  */
+		efer |= EFER_NX;
+
+		if (!(efer & EFER_LMA))
+			efer &= ~EFER_LME;
+	}
 
 	to_svm(vcpu)->vmcb->save.efer = efer | EFER_SVME;
 	mark_dirty(to_svm(vcpu)->vmcb, VMCB_CR);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 2a2ba277c676..e191d41afb34 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -896,17 +896,9 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 	u64 guest_efer = vmx->vcpu.arch.efer;
 	u64 ignore_bits = 0;
 
-	if (!enable_ept) {
-		/*
-		 * NX is needed to handle CR0.WP=1, CR4.SMEP=1.  Testing
-		 * host CPUID is more efficient than testing guest CPUID
-		 * or CR4.  Host SMEP is anyway a requirement for guest SMEP.
-		 */
-		if (boot_cpu_has(X86_FEATURE_SMEP))
-			guest_efer |= EFER_NX;
-		else if (!(guest_efer & EFER_NX))
-			ignore_bits |= EFER_NX;
-	}
+	/* Shadow paging assumes NX to be available.  */
+	if (!enable_ept)
+		guest_efer |= EFER_NX;
 
 	/*
 	 * LMA and LME handled by hardware; SCE meaningless outside long mode.
-- 
2.21.0