KVM: x86: Rename APIC base setters to better capture their relationship

Rename kvm_set_apic_base() and kvm_lapic_set_base() to kvm_apic_set_base()
and __kvm_apic_set_base() respectively to capture that the underscores
version is a "special" variant (it exists purely to avoid recalculating
the optimized map multiple times when stuffing the RESET value).

Opportunistically add a comment explaining why kvm_lapic_reset() uses the
inner helper.  Note, KVM deliberately invokes kvm_arch_vcpu_create() while
kvm->lock is NOT held so that vCPU setup isn't serialized if userspace is
creating multiple/all vCPUs in parallel.  I.e. triggering an extra
recalculation is not limited to theoretical/rare edge cases, and so is
worth avoiding.

No functional change intended.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20241009181742.1128779-7-seanjc@google.com
Link: https://lore.kernel.org/r/20241101183555.1794700-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/lapic.c

@@ -2577,7 +2577,7 @@ u64 kvm_lapic_get_cr8(struct kvm_vcpu *vcpu)
 	return (tpr & 0xf0) >> 4;
 }
 
-void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
+static void __kvm_apic_set_base(struct kvm_vcpu *vcpu, u64 value)
 {
 	u64 old_value = vcpu->arch.apic_base;
 	struct kvm_lapic *apic = vcpu->arch.apic;
@@ -2628,7 +2628,7 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
 	}
 }
 
-int kvm_set_apic_base(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
+int kvm_apic_set_base(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 {
 	enum lapic_mode old_mode = kvm_get_apic_mode(vcpu);
 	enum lapic_mode new_mode = kvm_apic_mode(msr_info->data);
@@ -2644,7 +2644,7 @@ int kvm_set_apic_base(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			return 1;
 	}
 
-	kvm_lapic_set_base(vcpu, msr_info->data);
+	__kvm_apic_set_base(vcpu, msr_info->data);
 	kvm_recalculate_apic_map(vcpu->kvm);
 	return 0;
 }
@@ -2752,7 +2752,14 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
 		msr_val = APIC_DEFAULT_PHYS_BASE | MSR_IA32_APICBASE_ENABLE;
 		if (kvm_vcpu_is_reset_bsp(vcpu))
 			msr_val |= MSR_IA32_APICBASE_BSP;
-		kvm_lapic_set_base(vcpu, msr_val);
+
+		/*
+		 * Use the inner helper to avoid an extra recalcuation of the
+		 * optimized APIC map if some other task has dirtied the map.
+		 * The recalculation needed for this vCPU will be done after
+		 * all APIC state has been initialized (see below).
+		 */
+		__kvm_apic_set_base(vcpu, msr_val);
 	}
 
 	if (!apic)

arch/x86/kvm/lapic.h

@@ -95,7 +95,6 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event);
 u64 kvm_lapic_get_cr8(struct kvm_vcpu *vcpu);
 void kvm_lapic_set_tpr(struct kvm_vcpu *vcpu, unsigned long cr8);
 void kvm_lapic_set_eoi(struct kvm_vcpu *vcpu);
-void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value);
 void kvm_recalculate_apic_map(struct kvm *kvm);
 void kvm_apic_set_version(struct kvm_vcpu *vcpu);
 void kvm_apic_after_set_mcg_cap(struct kvm_vcpu *vcpu);
@@ -117,7 +116,7 @@ bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm, struct kvm_lapic *src,
 		struct kvm_lapic_irq *irq, int *r, struct dest_map *dest_map);
 void kvm_apic_send_ipi(struct kvm_lapic *apic, u32 icr_low, u32 icr_high);
 
-int kvm_set_apic_base(struct kvm_vcpu *vcpu, struct msr_data *msr_info);
+int kvm_apic_set_base(struct kvm_vcpu *vcpu, struct msr_data *msr_info);
 int kvm_apic_get_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s);
 int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s);
 int kvm_lapic_find_highest_irr(struct kvm_vcpu *vcpu);

arch/x86/kvm/x86.c

@@ -3863,7 +3863,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_MTRRdefType:
 		return kvm_mtrr_set_msr(vcpu, msr, data);
 	case MSR_IA32_APICBASE:
-		return kvm_set_apic_base(vcpu, msr_info);
+		return kvm_apic_set_base(vcpu, msr_info);
 	case APIC_BASE_MSR ... APIC_BASE_MSR + 0xff:
 		return kvm_x2apic_msr_write(vcpu, msr, data);
 	case MSR_IA32_TSC_DEADLINE:
@@ -11879,7 +11879,7 @@ static int __set_sregs_common(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs,
 
 	apic_base_msr.data = sregs->apic_base;
 	apic_base_msr.host_initiated = true;
-	if (kvm_set_apic_base(vcpu, &apic_base_msr))
+	if (kvm_apic_set_base(vcpu, &apic_base_msr))
 		return -EINVAL;
 
 	if (vcpu->arch.guest_state_protected)