KVM: Calculate available entries in coalesced mmio ring
Instead of checking whether we'll wrap around, calculate how many entries
are available, and check whether we have enough (just one) for the pending
mmio.
By itself, this doesn't change anything, but it paves the way for making
this function lockless.
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index c4c7ec2..7549068 100644
--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -24,7 +24,8 @@
{
struct kvm_coalesced_mmio_dev *dev = to_mmio(this);
struct kvm_coalesced_mmio_zone *zone;
- int next;
+ struct kvm_coalesced_mmio_ring *ring;
+ unsigned avail;
int i;
if (!is_write)
@@ -40,10 +41,9 @@
* check if we don't meet the first used entry
* there is always one unused entry in the buffer
*/
-
- next = (dev->kvm->coalesced_mmio_ring->last + 1) %
- KVM_COALESCED_MMIO_MAX;
- if (next == dev->kvm->coalesced_mmio_ring->first) {
+ ring = dev->kvm->coalesced_mmio_ring;
+ avail = (ring->first - ring->last - 1) % KVM_COALESCED_MMIO_MAX;
+ if (avail < 1) {
/* full */
return 0;
}