linux-kernel - [PATCH 2/2] KVM: x86: nVMX: fix x2APIC VTPR read intercept

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Date:   Sat,  6 Apr 2019 00:46:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Marc Orr <marcorr@...gle.com>
Subject: [PATCH 2/2] KVM: x86: nVMX: fix x2APIC VTPR read intercept

From: Marc Orr <marcorr@...gle.com>

Referring to the "VIRTUALIZING MSR-BASED APIC ACCESSES" chapter of the
SDM, when "virtualize x2APIC mode" is 1 and "APIC-register
virtualization" is 0, a RDMSR of 808H should return the VTPR from the
virtual APIC page.

However, for nested, KVM currently fails to disable the read intercept
for this MSR. This means that a RDMSR exit takes precedence over
"virtualize x2APIC mode", and KVM passes through L1's TPR to L2,
instead of sourcing the value from L2's virtual APIC page.

This patch fixes the issue by disabling the read intercept, in VMCS02,
for the VTPR when "APIC-register virtualization" is 0.

The issue described above and fix prescribed here, were verified with
a related patch in kvm-unit-tests titled "Test VMX's virtualize x2APIC
mode w/ nested".

Signed-off-by: Marc Orr <marcorr@...gle.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>
Fixes: c992384bde84f ("KVM: vmx: speed up MSR bitmap merge")
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/vmx/nested.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 897d70e3d291..7ec9bb1dd723 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -578,7 +578,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
 		nested_vmx_disable_intercept_for_msr(
 			msr_bitmap_l1, msr_bitmap_l0,
 			X2APIC_MSR(APIC_TASKPRI),
-			MSR_TYPE_W);
+			MSR_TYPE_R | MSR_TYPE_W);

 		if (nested_cpu_has_vid(vmcs12)) {
 			nested_vmx_disable_intercept_for_msr(
-- 
1.8.3.1