Index: head/sys/dev/ixgbe/ix_txrx.c
===================================================================
--- head/sys/dev/ixgbe/ix_txrx.c
+++ head/sys/dev/ixgbe/ix_txrx.c
@@ -297,6 +297,8 @@
 	ntxd = scctx->isc_ntxd[0];
 	do {
 		delta = (int32_t)cur - (int32_t)prev;
+		if (prev == 0 && cur == 0)
+			delta += 1;
 		if (delta < 0)
 			delta += ntxd;
 
Index: head/sys/dev/ixl/ixl_txrx.c
===================================================================
--- head/sys/dev/ixl/ixl_txrx.c
+++ head/sys/dev/ixl/ixl_txrx.c
@@ -516,7 +516,13 @@
 	ntxd = scctx->isc_ntxd[0];
 	do {
 		delta = (int32_t)cur - (int32_t)prev;
+		/*
+		 * XXX This appears to be a hack for first-packet.
+		 * A correct fix would prevent prev == cur in the first place.
+		 */
 		MPASS(prev == 0 || delta != 0);
+		if (prev == 0 && cur == 0)
+			delta += 1;
 		if (delta < 0)
 			delta += ntxd;
 #if 0