[PATCH 3/4] KVM: gracefully handle zero in kvm_free_irq_source_id()

[Mark McLoughlin <markmc@redhat.com>]

Allow kvm_free_irq_source_id() to be called with a zero ID.

Zero is reserved for KVM_USERSPACE_IRQ_SOURCE_ID, so we can
guarantee that kvm_request_irq_source_id() will never return
zero and use zero to indicate "no source ID allocated".

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
---
 virt/kvm/irq_comm.c |   14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
index db75045..869f5e8 100644
--- a/virt/kvm/irq_comm.c
+++ b/virt/kvm/irq_comm.c
@@ -72,11 +72,15 @@ int kvm_request_irq_source_id(struct kvm *kvm)
 	unsigned long *bitmap = &kvm->arch.irq_sources_bitmap;
 	int irq_source_id = find_first_zero_bit(bitmap,
 				sizeof(kvm->arch.irq_sources_bitmap));
+
 	if (irq_source_id >= sizeof(kvm->arch.irq_sources_bitmap)) {
 		printk(KERN_WARNING "kvm: exhaust allocatable IRQ sources!\n");
-		irq_source_id = -EFAULT;
-	} else
-		set_bit(irq_source_id, bitmap);
+		return -EFAULT;
+	}
+
+	ASSERT(irq_source_id != 0); /* KVM_USERSPACE_IRQ_SOURCE_ID reserved */
+	set_bit(irq_source_id, bitmap);
+
 	return irq_source_id;
 }
 
@@ -84,7 +88,9 @@ void kvm_free_irq_source_id(struct kvm *kvm, int irq_source_id)
 {
 	int i;
 
-	if (irq_source_id <= 0 ||
+	if (!irq_source_id)
+		return;
+	if (irq_source_id < 0 ||
 	    irq_source_id >= sizeof(kvm->arch.irq_sources_bitmap)) {
 		printk(KERN_ERR "kvm: IRQ source ID out of range!\n");
 		return;
-- 
1.5.4.3