linux-kernel - [PATCH] KVM: SVM: Do not virtualize MSR accesses for APIC LVTT register

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Date:   Sun, 24 Jul 2022 22:34:28 -0500
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
CC:     <pbonzini@...hat.com>, <mlevitsk@...hat.com>, <seanjc@...gle.com>,
        <jon.grimm@....com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Subject: [PATCH] KVM: SVM: Do not virtualize MSR accesses for APIC LVTT register

AMD does not support APIC TSC-deadline timer mode. AVIC hardware
will generate GP fault when guest kernel writes 1 to bits [18]
of the APIC LVTT register (offset 0x32) to set the timer mode.
(Note: bit 18 is reserved on AMD system).

Therefore, always intercept and let KVM emulate the MSR accesses.

Fixes: f3d7c8aa6882 ("KVM: SVM: Fix x2APIC MSRs interception")
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
---
 arch/x86/kvm/svm/svm.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index aef63aae922d..3e0639a68385 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -118,7 +118,14 @@ static const struct svm_direct_access_msrs {
 	{ .index = X2APIC_MSR(APIC_ESR),		.always = false },
 	{ .index = X2APIC_MSR(APIC_ICR),		.always = false },
 	{ .index = X2APIC_MSR(APIC_ICR2),		.always = false },
-	{ .index = X2APIC_MSR(APIC_LVTT),		.always = false },
+
+	/*
+	 * Note:
+	 * AMD does not virtualize APIC TSC-deadline timer mode, but it is
+	 * emulated by KVM. When setting APIC LVTT (0x832) register bit 18,
+	 * the AVIC hardware would generate GP fault. Therefore, always
+	 * intercept the MSR 0x832, and do not setup direct_access_msr.
+	 */
 	{ .index = X2APIC_MSR(APIC_LVTTHMR),		.always = false },
 	{ .index = X2APIC_MSR(APIC_LVTPC),		.always = false },
 	{ .index = X2APIC_MSR(APIC_LVT0),		.always = false },
-- 
2.34.1