linux-kernel - [RFCv2 PATCH 06/12] KVM: SVM: Do not update logical APIC ID table when in x2APIC mode

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Message-ID: <20220308163926.563994-7-suravee.suthikulpanit@amd.com>
Date:   Tue, 8 Mar 2022 10:39:20 -0600
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>,
        <joro@...tes.org>, <jon.grimm@....com>, <wei.huang2@....com>,
        <terry.bowman@....com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Subject: [RFCv2 PATCH 06/12] KVM: SVM: Do not update logical APIC ID table when in x2APIC mode

In X2APIC mode the Logical Destination Register is read-only,
which provides a fixed mapping between the logical and physical
APIC IDs. Therefore, there is no Logical APIC ID table in X2AVIC
and the processor uses the X2APIC ID in the backing page to create
a vCPU’s logical ID. Also, when x2AVIC is activated, a guest write
to the x2APIC LDR register would result in #GP injection into
the guest by the hardware.

Therefore, add logic to check x2APIC mode before updating logical
APIC ID table.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
---
 arch/x86/kvm/svm/avic.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 5329b93dc4cd..4d7a8743196e 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -406,6 +406,10 @@ static int avic_ldr_write(struct kvm_vcpu *vcpu, u8 g_physical_id, u32 ldr)
 	bool flat;
 	u32 *entry, new_entry;
 
+	/* Note: x2AVIC does not use logical APIC ID table */
+	if (apic_x2apic_mode(vcpu->arch.apic))
+		return 0;
+
 	flat = kvm_lapic_get_reg(vcpu->arch.apic, APIC_DFR) == APIC_DFR_FLAT;
 	entry = avic_get_logical_id_entry(vcpu, ldr, flat);
 	if (!entry)
@@ -424,8 +428,13 @@ static void avic_invalidate_logical_id_entry(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 	bool flat = svm->dfr_reg == APIC_DFR_FLAT;
-	u32 *entry = avic_get_logical_id_entry(vcpu, svm->ldr_reg, flat);
+	u32 *entry;
+
+	/* Note: x2AVIC does not use logical APIC ID table */
+	if (apic_x2apic_mode(vcpu->arch.apic))
+		return;
 
+	entry = avic_get_logical_id_entry(vcpu, svm->ldr_reg, flat);
 	if (entry)
 		clear_bit(AVIC_LOGICAL_ID_ENTRY_VALID_BIT, (unsigned long *)entry);
 }
-- 
2.25.1