[PATCH] KVM: x86: Allow APICv APIC ID inhibit to be cleared on legacy kernels

[Greg Edwards]

Legacy kernels prior to commit 4399c03c6780 ("x86/apic: Remove
verify_local_APIC()") write the xAPIC ID of the boot CPU twice to verify
a functioning local APIC.  This results in APIC acceleration inhibited
on these kernels for reason APICV_INHIBIT_REASON_APIC_ID_MODIFIED.

Allow the APICV_INHIBIT_REASON_APIC_ID_MODIFIED inhibit reason to be
cleared if/when the xAPIC ID is set back to the expected vcpu_id value.
This occurs on the second xAPIC ID write in verify_local_APIC().

Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
Signed-off-by: Greg Edwards <gedwards@xxxxxxx>
---
 arch/x86/kvm/lapic.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index d7639d126e6c..4064d0af094d 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2075,8 +2075,13 @@ static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
 	if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm))
 		return;
 
-	if (kvm_xapic_id(apic) == apic->vcpu->vcpu_id)
+	if (kvm_xapic_id(apic) == apic->vcpu->vcpu_id) {
+		/* Legacy kernels prior to 4399c03c6780 write APIC ID twice. */
+		if (!kvm_apicv_activated(kvm))
+			kvm_clear_apicv_inhibit(kvm,
+					APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
 		return;
+	}
 
 	kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
 }
-- 
2.38.1