[PATCH v2] KVM: x86: Allow APICv APIC ID inhibit to be cleared

[Greg Edwards]

Legacy kernels prior to commit 4399c03c6780 ("x86/apic: Remove
verify_local_APIC()") write the APIC 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 all APICs in xAPIC mode set their APIC ID back to the
expected vcpu_id value.

Fold the functionality previously in kvm_lapic_xapic_id_updated() into
kvm_recalculate_apic_map(), as this allows us examine all APICs in one
pass.

Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
Signed-off-by: Greg Edwards <gedwards@xxxxxxx>
---
Changes from v1:
  * Fold kvm_lapic_xapic_id_updated() into kvm_recalculate_apic_map() and
    verify no APICs in xAPIC mode have a modified APIC ID before clearing
    APICV_INHIBIT_REASON_APIC_ID_MODIFIED.  [Sean]
  * Rebase on top of Sean's APIC fixes+cleanups series.  [Sean]
    https://lore.kernel.org/all/20221001005915.2041642-1-seanjc@xxxxxxxxxx/

 arch/x86/kvm/lapic.c | 45 +++++++++++++++++++-------------------------
 1 file changed, 19 insertions(+), 26 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 9b3af49d2524..362472da6e7f 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -236,6 +236,7 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
 	struct kvm_vcpu *vcpu;
 	unsigned long i;
 	u32 max_id = 255; /* enough space for any xAPIC ID */
+	bool xapic_id_modified = false;
 
 	/* Read kvm->arch.apic_map_dirty before kvm->arch.apic_map.  */
 	if (atomic_read_acquire(&kvm->arch.apic_map_dirty) == CLEAN)
@@ -285,6 +286,19 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
 		xapic_id = kvm_xapic_id(apic);
 		x2apic_id = kvm_x2apic_id(apic);
 
+		if (!apic_x2apic_mode(apic)) {
+			/*
+			 * Deliberately truncate the vCPU ID when detecting a
+			 * modified APIC ID to avoid false positives if the
+			 * vCPU ID, i.e. x2APIC ID, is a 32-bit value.  If the
+			 * wrap/truncation results in unwanted aliasing, APICv
+			 * will be inhibited as part of updating KVM's
+			 * optimized APIC maps.
+			 */
+			if (xapic_id != (u8)x2apic_id)
+				xapic_id_modified = true;
+		}
+
 		/*
 		 * Apply KVM's hotplug hack if userspace has enable 32-bit APIC
 		 * IDs.  Allow sending events to vCPUs by their x2APIC ID even
@@ -396,6 +410,11 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
 	else
 		kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED);
 
+	if (xapic_id_modified)
+		kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
+	else
+		kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
+
 	old = rcu_dereference_protected(kvm->arch.apic_map,
 			lockdep_is_held(&kvm->arch.apic_map_lock));
 	rcu_assign_pointer(kvm->arch.apic_map, new);
@@ -2155,28 +2174,6 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
 	}
 }
 
-static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
-{
-	struct kvm *kvm = apic->vcpu->kvm;
-
-	if (!kvm_apic_hw_enabled(apic))
-		return;
-
-	if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm))
-		return;
-
-	/*
-	 * Deliberately truncate the vCPU ID when detecting a modified APIC ID
-	 * to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit
-	 * value.  If the wrap/truncation results in unwatned aliasing, APICv
-	 * will be inhibited as part of updating KVM's optimized APIC maps.
-	 */
-	if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id)
-		return;
-
-	kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
-}
-
 static int get_lvt_index(u32 reg)
 {
 	if (reg == APIC_LVTCMCI)
@@ -2197,7 +2194,6 @@ static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 	case APIC_ID:		/* Local APIC ID */
 		if (!apic_x2apic_mode(apic)) {
 			kvm_apic_set_xapic_id(apic, val >> 24);
-			kvm_lapic_xapic_id_updated(apic);
 		} else {
 			ret = 1;
 		}
@@ -2919,9 +2915,6 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
 	}
 	memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
 
-	if (!apic_x2apic_mode(vcpu->arch.apic))
-		kvm_lapic_xapic_id_updated(vcpu->arch.apic);
-
 	atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
 	kvm_recalculate_apic_map(vcpu->kvm);
 	kvm_apic_set_version(vcpu);
-- 
2.38.1