add m_copyup function.. This can be used to help make our ip stack less

alignment restrictive, and help performance on some ethernet cards which
currently copy the entire packet a couple bytes to get the packet aligned
properly...

Wordsmithing by:	dwhite
Obtained from:	NetBSD (code only)
I'll clean it up later:	rwatson

share/man/man9/mbuf.9

@@ -95,6 +95,8 @@
 .Ft struct mbuf *
 .Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
 .Ft struct mbuf *
+.Fn m_copyup "struct mbuf *mbuf" "int len" "int dstoff"
+.Ft struct mbuf *
 .Fn m_pullup "struct mbuf *mbuf" "int len"
 .Ft struct mbuf *
 .Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
@@ -614,6 +616,33 @@ depending on the
 .Dv M_PKTHDR
 flag setting.
 .\"
+.It Fn m_copyup mbuf len dstoff
+Similar to
+.Fn m_pullup
+but copies
+.Fa len
+bytes of data into a new mbuf at
+.Fa dstoff
+bytes into the mbuf.
+The
+.Fa dstoff
+argument aligns the data and leaves room for a link layer header.
+Return the new
+.Vt mbuf chain
+on success,
+and frees the
+.Vt mbuf chain
+and returns
+.Dv NULL
+on failure.
+.Sy Note :
+The function does not allocate
+.Vt mbuf clusters ,
+so
+.Fa len + dstoff
+must be less than
+.Dv MHLEN .
+.\"
 .It Fn m_pullup mbuf len
 Arrange that the first
 .Fa len

sys/kern/uipc_mbuf.c

@@ -766,6 +766,54 @@ m_pullup(struct mbuf *n, int len)
 	return (NULL);
 }
 
+/*
+ * Like m_pullup(), except a new mbuf is always allocated, and we allow
+ * the amount of empty space before the data in the new mbuf to be specified
+ * (in the event that the caller expects to prepend later).
+ */
+int MSFail;
+
+struct mbuf *
+m_copyup(struct mbuf *n, int len, int dstoff)
+{
+	struct mbuf *m;
+	int count, space;
+
+	if (len > (MHLEN - dstoff))
+		goto bad;
+	MGET(m, M_DONTWAIT, n->m_type);
+	if (m == NULL)
+		goto bad;
+	m->m_len = 0;
+	if (n->m_flags & M_PKTHDR)
+		M_MOVE_PKTHDR(m, n);
+	m->m_data += dstoff;
+	space = &m->m_dat[MLEN] - (m->m_data + m->m_len);
+	do {
+		count = min(min(max(len, max_protohdr), space), n->m_len);
+		memcpy(mtod(m, caddr_t) + m->m_len, mtod(n, caddr_t),
+		    (unsigned)count);
+		len -= count;
+		m->m_len += count;
+		n->m_len -= count;
+		space -= count;
+		if (n->m_len)
+			n->m_data += count;
+		else
+			n = m_free(n);
+	} while (len > 0 && n);
+	if (len > 0) {
+		(void) m_free(m);
+		goto bad;
+	}
+	m->m_next = n;
+	return (m);
+ bad:
+	m_freem(n);
+	MSFail++;
+	return (NULL);
+}
+
 /*
  * Partition an mbuf chain in two pieces, returning the tail --
  * all but the first len0 bytes.  In case of failure, it returns NULL and

sys/sys/mbuf.h

@@ -564,6 +564,7 @@ void		 m_copydata(const struct mbuf *, int, int, caddr_t);
 struct	mbuf	*m_copym(struct mbuf *, int, int, int);
 struct	mbuf	*m_copypacket(struct mbuf *, int);
 void		 m_copy_pkthdr(struct mbuf *, struct mbuf *);
+struct	mbuf	*m_copyup(struct mbuf *n, int len, int dstoff);
 struct	mbuf	*m_defrag(struct mbuf *, int);
 struct	mbuf	*m_devget(char *, int, int, struct ifnet *,
 		    void (*)(char *, caddr_t, u_int));