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

Tyler Hicks tyhicks at canonical.com
Thu Apr 18 07:33:34 UTC 2019

From: Marc Orr <marcorr at google.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 at google.com>
Reviewed-by: Jim Mattson <jmattson at google.com>
Fixes: c992384bde84f ("KVM: vmx: speed up MSR bitmap merge")
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>


(cherry picked from commit c73f4c998e1fd4249b9edfa39e23f4fda2b9b041)
Signed-off-by: Tyler Hicks <tyhicks at canonical.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 7c1be5a139a7..f90b3a948291 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,
 			msr_bitmap_l1, msr_bitmap_l0,
-			MSR_TYPE_W);
 		if (nested_cpu_has_vid(vmcs12)) {

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