KVM: x86: Allow APICv APIC ID inhibit to be cleared

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 examining 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@ddn.com>
Link: https://lore.kernel.org/r/20221117183247.94314-1-gedwards@ddn.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230106011306.85230-33-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 0faf80c..856e81a 100644 (file)
--- 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_mismatch = false;
 
        /* Read kvm->arch.apic_map_dirty before kvm->arch.apic_map.  */
        if (atomic_read_acquire(&kvm->arch.apic_map_dirty) == CLEAN)
@@ -286,6 +287,15 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
                x2apic_id = kvm_x2apic_id(apic);
 
                /*
+                * Deliberately truncate the vCPU ID when detecting a mismatched
+                * APIC ID to avoid false positives if the vCPU ID, i.e. x2APIC
+                * ID, is a 32-bit value.  Any unwanted aliasing due to
+                * truncation results will be detected below.
+                */
+               if (!apic_x2apic_mode(apic) && xapic_id != (u8)vcpu->vcpu_id)
+                       xapic_id_mismatch = true;
+
+               /*
                 * Apply KVM's hotplug hack if userspace has enable 32-bit APIC
                 * IDs.  Allow sending events to vCPUs by their x2APIC ID even
                 * if the target vCPU is in legacy xAPIC mode, and silently
@@ -401,6 +411,11 @@ out:
        else
                kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED);
 
+       if (xapic_id_mismatch)
+               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);
@@ -2160,28 +2175,6 @@ 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 unwanted 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)
@@ -2202,7 +2195,6 @@ 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;
                }
@@ -2923,9 +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(apic))
-               kvm_lapic_xapic_id_updated(apic);
-
        atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
        kvm_recalculate_apic_map(vcpu->kvm);
        kvm_apic_set_version(vcpu);