svn commit: r247871 - head/usr.sbin/bhyve

[Peter Grehan]

Author: grehan
Date: Wed Mar  6 07:28:20 2013
New Revision: 247871
URL: http://svnweb.freebsd.org/changeset/base/247871

Log:
  Simplify virtio ring num-available calculation.
  
  Submitted by:	Chris Torek, torek at torek dot net

Modified:
  head/usr.sbin/bhyve/pci_virtio_block.c
  head/usr.sbin/bhyve/pci_virtio_net.c

Modified: head/usr.sbin/bhyve/pci_virtio_block.c
==============================================================================
--- head/usr.sbin/bhyve/pci_virtio_block.c	Wed Mar  6 07:17:53 2013	(r247870)
+++ head/usr.sbin/bhyve/pci_virtio_block.c	Wed Mar  6 07:28:20 2013	(r247871)
@@ -164,14 +164,19 @@ pci_vtblk_iosize(struct pci_devinst *pi)
 static int
 hq_num_avail(struct vring_hqueue *hq)
 {
-	int ndesc;
+	uint16_t ndesc;
 
-	if (*hq->hq_avail_idx >= hq->hq_cur_aidx)
-		ndesc = *hq->hq_avail_idx - hq->hq_cur_aidx;
-	else
-		ndesc = UINT16_MAX - hq->hq_cur_aidx + *hq->hq_avail_idx + 1;
+	/*
+	 * We're just computing (a-b) in GF(216).
+	 *
+	 * The only glitch here is that in standard C,
+	 * uint16_t promotes to (signed) int when int has
+	 * more than 16 bits (pretty much always now), so
+	 * we have to force it back to unsigned.
+	 */
+	ndesc = (unsigned)*hq->hq_avail_idx - (unsigned)hq->hq_cur_aidx;
 
-	assert(ndesc >= 0 && ndesc <= hq->hq_size);
+	assert(ndesc <= hq->hq_size);
 
 	return (ndesc);
 }

Modified: head/usr.sbin/bhyve/pci_virtio_net.c
==============================================================================
--- head/usr.sbin/bhyve/pci_virtio_net.c	Wed Mar  6 07:17:53 2013	(r247870)
+++ head/usr.sbin/bhyve/pci_virtio_net.c	Wed Mar  6 07:28:20 2013	(r247871)
@@ -172,12 +172,17 @@ hq_num_avail(struct vring_hqueue *hq)
 {
 	int ndesc;
 
-	if (*hq->hq_avail_idx >= hq->hq_cur_aidx)
-		ndesc = *hq->hq_avail_idx - hq->hq_cur_aidx;
-	else
-		ndesc = UINT16_MAX - hq->hq_cur_aidx + *hq->hq_avail_idx + 1;
+	/*
+	 * We're just computing (a-b) in GF(216).
+	 *
+	 * The only glitch here is that in standard C,
+	 * uint16_t promotes to (signed) int when int has
+	 * more than 16 bits (pretty much always now), so
+	 * we have to force it back to unsigned.
+	 */
+	ndesc = (unsigned)*hq->hq_avail_idx - (unsigned)hq->hq_cur_aidx;
 
-	assert(ndesc >= 0 && ndesc <= hq->hq_size);
+	assert(ndesc <= hq->hq_size);
 
 	return (ndesc);
 }