KVM: Allow building irqbypass.ko as as module when kvm.ko is a module

Convert HAVE_KVM_IRQ_BYPASS into a tristate so that selecting
IRQ_BYPASS_MANAGER follows KVM={m,y}, i.e. doesn't force irqbypass.ko to
be built-in.

Note, PPC allows building KVM as a module, but selects HAVE_KVM_IRQ_BYPASS
from a boolean Kconfig, i.e. KVM PPC unnecessarily forces irqbpass.ko to
be built-in.  But that flaw is a longstanding PPC specific issue.

Fixes: 61df71e ("kvm: move "select IRQ_BYPASS_MANAGER" to common code")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250315024623.2363994-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -2382,7 +2382,7 @@ static inline bool kvm_is_visible_memslot(struct kvm_memory_slot *memslot)
 struct kvm_vcpu *kvm_get_running_vcpu(void);
 struct kvm_vcpu * __percpu *kvm_get_running_vcpus(void);
 
-#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
+#if IS_ENABLED(CONFIG_HAVE_KVM_IRQ_BYPASS)
 bool kvm_arch_has_irq_bypass(void);
 int kvm_arch_irq_bypass_add_producer(struct irq_bypass_consumer *,
 			   struct irq_bypass_producer *);

virt/kvm/Kconfig

@@ -75,7 +75,7 @@ config KVM_COMPAT
        depends on KVM && COMPAT && !(S390 || ARM64 || RISCV)
 
 config HAVE_KVM_IRQ_BYPASS
-       bool
+       tristate
        select IRQ_BYPASS_MANAGER
 
 config HAVE_KVM_VCPU_ASYNC_IOCTL

virt/kvm/eventfd.c

@@ -149,7 +149,7 @@ irqfd_shutdown(struct work_struct *work)
 	/*
 	 * It is now safe to release the object's resources
 	 */
-#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
+#if IS_ENABLED(CONFIG_HAVE_KVM_IRQ_BYPASS)
 	irq_bypass_unregister_consumer(&irqfd->consumer);
 #endif
 	eventfd_ctx_put(irqfd->eventfd);
@@ -274,7 +274,7 @@ static void irqfd_update(struct kvm *kvm, struct kvm_kernel_irqfd *irqfd)
 	write_seqcount_end(&irqfd->irq_entry_sc);
 }
 
-#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
+#if IS_ENABLED(CONFIG_HAVE_KVM_IRQ_BYPASS)
 void __attribute__((weak)) kvm_arch_irq_bypass_stop(
 				struct irq_bypass_consumer *cons)
 {
@@ -424,7 +424,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 	if (events & EPOLLIN)
 		schedule_work(&irqfd->inject);
 
-#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
+#if IS_ENABLED(CONFIG_HAVE_KVM_IRQ_BYPASS)
 	if (kvm_arch_has_irq_bypass()) {
 		irqfd->consumer.token = (void *)irqfd->eventfd;
 		irqfd->consumer.add_producer = kvm_arch_irq_bypass_add_producer;
@@ -609,14 +609,14 @@ void kvm_irq_routing_update(struct kvm *kvm)
 	spin_lock_irq(&kvm->irqfds.lock);
 
 	list_for_each_entry(irqfd, &kvm->irqfds.items, list) {
-#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
+#if IS_ENABLED(CONFIG_HAVE_KVM_IRQ_BYPASS)
 		/* Under irqfds.lock, so can read irq_entry safely */
 		struct kvm_kernel_irq_routing_entry old = irqfd->irq_entry;
 #endif
 
 		irqfd_update(kvm, irqfd);
 
-#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
+#if IS_ENABLED(CONFIG_HAVE_KVM_IRQ_BYPASS)
 		if (irqfd->producer &&
 		    kvm_arch_irqfd_route_changed(&old, &irqfd->irq_entry)) {
 			int ret = kvm_arch_update_irqfd_routing(