KVM: x86: dynamic kvm_apic_map

x2APIC supports up to 2^32-1 LAPICs, but most guest in coming years will
probably has fewer VCPUs.  Dynamic size saves memory at the cost of
turning one constant into a variable.

apic_map mutex had to be moved before allocation to avoid races with cpu
hotplug.
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -682,11 +682,12 @@ struct kvm_arch_memory_slot {
 struct kvm_apic_map {
 	struct rcu_head rcu;
 	u8 mode;
-	struct kvm_lapic *phys_map[256];
+	u32 max_apic_id;
 	union {
 		struct kvm_lapic *xapic_flat_map[8];
 		struct kvm_lapic *xapic_cluster_map[16][4];
 	};
+	struct kvm_lapic *phys_map[];
 };
 
 /* Hyper-V emulation context */

arch/x86/kvm/lapic.c

@@ -120,7 +120,7 @@ static inline bool kvm_apic_map_get_logical_dest(struct kvm_apic_map *map,
 	switch (map->mode) {
 	case KVM_APIC_MODE_X2APIC: {
 		u32 offset = (dest_id >> 16) * 16;
-		u32 max_apic_id = ARRAY_SIZE(map->phys_map) - 1;
+		u32 max_apic_id = map->max_apic_id;
 
 		if (offset <= max_apic_id) {
 			u8 cluster_size = min(max_apic_id - offset + 1, 16U);
@@ -152,14 +152,22 @@ static void recalculate_apic_map(struct kvm *kvm)
 	struct kvm_apic_map *new, *old = NULL;
 	struct kvm_vcpu *vcpu;
 	int i;
-
-	new = kzalloc(sizeof(struct kvm_apic_map), GFP_KERNEL);
+	u32 max_id = 255;
 
 	mutex_lock(&kvm->arch.apic_map_lock);
 
+	kvm_for_each_vcpu(i, vcpu, kvm)
+		if (kvm_apic_present(vcpu))
+			max_id = max(max_id, kvm_apic_id(vcpu->arch.apic));
+
+	new = kzalloc(sizeof(struct kvm_apic_map) +
+	              sizeof(struct kvm_lapic *) * (max_id + 1), GFP_KERNEL);
+
 	if (!new)
 		goto out;
 
+	new->max_apic_id = max_id;
+
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		struct kvm_lapic *apic = vcpu->arch.apic;
 		struct kvm_lapic **cluster;
@@ -172,7 +180,7 @@ static void recalculate_apic_map(struct kvm *kvm)
 		aid = kvm_apic_id(apic);
 		ldr = kvm_lapic_get_reg(apic, APIC_LDR);
 
-		if (aid < ARRAY_SIZE(new->phys_map))
+		if (aid <= new->max_apic_id)
 			new->phys_map[aid] = apic;
 
 		if (apic_x2apic_mode(apic)) {
@@ -710,7 +718,7 @@ static inline bool kvm_apic_map_get_dest_lapic(struct kvm *kvm,
 		return false;
 
 	if (irq->dest_mode == APIC_DEST_PHYSICAL) {
-		if (irq->dest_id >= ARRAY_SIZE(map->phys_map)) {
+		if (irq->dest_id > map->max_apic_id) {
 			*bitmap = 0;
 		} else {
 			*dst = &map->phys_map[irq->dest_id];

arch/x86/kvm/lapic.h

@@ -200,7 +200,7 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
 	return lapic_in_kernel(vcpu) && test_bit(KVM_APIC_INIT, &vcpu->arch.apic->pending_events);
 }
 
-static inline int kvm_apic_id(struct kvm_lapic *apic)
+static inline u32 kvm_apic_id(struct kvm_lapic *apic)
 {
 	return (kvm_lapic_get_reg(apic, APIC_ID) >> 24) & 0xff;
 }