KVM: Clean up coalesced MMIO ring full check

Fold coalesced_mmio_has_room() into its sole caller, coalesced_mmio_write(),
as it's really just a single line of code, has a goofy return value, and
is unnecessarily brittle.

E.g. if coalesced_mmio_has_room() were to check ring->last directly, or
the caller failed to use READ_ONCE(), KVM would be susceptible to TOCTOU
attacks from userspace.

Opportunistically add a comment explaining why on earth KVM leaves one
entry free, which may not be obvious to readers that aren't familiar with
ring buffers.

No functional change intended.
Reviewed-by: Ilias Stamatis <ilstam@amazon.com>
Cc: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240828181446.652474-3-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

virt/kvm/coalesced_mmio.c

@@ -40,25 +40,6 @@ static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev,
 	return 1;
 }
 
-static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev, u32 last)
-{
-	struct kvm_coalesced_mmio_ring *ring;
-
-	/* Are we able to batch it ? */
-
-	/* last is the first free entry
-	 * check if we don't meet the first used entry
-	 * there is always one unused entry in the buffer
-	 */
-	ring = dev->kvm->coalesced_mmio_ring;
-	if ((last + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) {
-		/* full */
-		return 0;
-	}
-
-	return 1;
-}
-
 static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
 				struct kvm_io_device *this, gpa_t addr,
 				int len, const void *val)
@@ -72,9 +53,15 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
 
 	spin_lock(&dev->kvm->ring_lock);
 
+	/*
+	 * last is the index of the entry to fill.  Verify userspace hasn't
+	 * set last to be out of range, and that there is room in the ring.
+	 * Leave one entry free in the ring so that userspace can differentiate
+	 * between an empty ring and a full ring.
+	 */
 	insert = READ_ONCE(ring->last);
-	if (!coalesced_mmio_has_room(dev, insert) ||
-	    insert >= KVM_COALESCED_MMIO_MAX) {
+	if (insert >= KVM_COALESCED_MMIO_MAX ||
+	    (insert + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) {
 		spin_unlock(&dev->kvm->ring_lock);
 		return -EOPNOTSUPP;
 	}