Index: head/sys/dev/hyperv/vmbus/vmbus_brvar.h
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus_brvar.h
+++ head/sys/dev/hyperv/vmbus/vmbus_brvar.h
@@ -74,8 +74,12 @@
 static __inline int
 vmbus_txbr_maxpktsz(const struct vmbus_txbr *tbr)
 {
-	/* 1/2 data size */
-	return (tbr->txbr_dsize / 2);
+	/*
+	 * - 64 bits for the trailing start index (- sizeof(uint64_t)).
+	 * - The rindex and windex can't be same (- 1).  See
+	 *   the comment near vmbus_bufring.br_{r,w}index.
+	 */
+	return (tbr->txbr_dsize - sizeof(uint64_t) - 1);
 }
 
 void		vmbus_br_sysctl_create(struct sysctl_ctx_list *ctx,