Make PKB_ZLEN the minimum possible size of packet buffer (to allow for

hardware that can't autopad).

src/include/gpxe/pkbuff.h

  */
 #define PKBUFF_ALIGN 2048
 
+/**
+ * Minimum packet buffer length
+ *
+ * alloc_pkb() will round up the allocated length to this size if
+ * necessary.  This is used on behalf of hardware that is not capable
+ * of auto-padding.
+ */
+#define PKB_ZLEN 64
+
 /** A packet buffer
  *
  * This structure is used to represent a network packet within gPXE.

src/net/pkbuff.c

 	struct pk_buff *pkb = NULL;
 	void *data;
 
+	/* Pad to minimum length */
+	if ( len < PKB_ZLEN )
+		len = PKB_ZLEN;
+
 	/* Align buffer length */
 	len = ( len + __alignof__( *pkb ) - 1 ) & ~( __alignof__( *pkb ) - 1 );