linux-kernel - [PATCH] KVM: x86: Un-inhibit APICv/AVIC when switching to x2APIC mode

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Date:   Fri, 9 Sep 2022 14:54:42 -0500
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
CC:     <pbonzini@...hat.com>, <jon.grimm@....com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH] KVM: x86: Un-inhibit APICv/AVIC when switching to x2APIC mode

Currently, kvm_lapic_xapic_id_updated() checks if the xAPIC ID in
the APIC_ID register differs from the vcpu ID. If so it inhibits
APICv/AVIC.

However, for vcpu 256 and above, the 8-bit xAPIC ID field in the APIC_ID
register cannot support 32-bit x2APIC ID causing the kvm_xapic_id()
to return invalid ID and fail the comparison with the vcpu ID.
This causes APICv/AVIC inhibition for VM with more than 256 vcpus
due to APIVC_INHIBIT_REASON_APIC_ID_MODIFIED.

In this case, when guest switch to x2APIC mode, KVM needs to clear
APIVC_INHIBIT_REASON_APIC_ID_MODIFIED.

Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
Cc: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
---
 arch/x86/kvm/lapic.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 9dda989a1cf0..750d385b770e 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -374,6 +374,7 @@ static inline void kvm_apic_set_x2apic_id(struct kvm_lapic *apic, u32 id)
 	kvm_lapic_set_reg(apic, APIC_ID, id);
 	kvm_lapic_set_reg(apic, APIC_LDR, ldr);
 	atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
+	kvm_clear_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
 }

 static inline int apic_lvt_enabled(struct kvm_lapic *apic, int lvt_type)
-- 
2.34.1