Allocate four bits from the mbuf flags field to represent the hash
type of a software- or hardware-generated hash held in the
mbuf.m_pkthdr.flowid field, and provide accessor macros to easily
clear, set, receive, and test for hash values.  Some of these
constants correspond to RSS hash types, but we don't want to limit
ourselves to that, as a number of other hashing techniques are in
use on hardware supported by FreeBSD.

Mark the M_FLOWID flag as deprecated; I hope to remove this before
9.0, changing drivers and the stack over to using the new
M_HASHTYPEBITS, most likely to use M_HASHTYPE_OPAQUE as we don't yet
want to nail down the KPI for RSS key/bucket management for device
drivers.

MFC after:      3 days
Reviewed by:    bz
Sponsored by:   Juniper Networks, Inc.

Index: sys/mbuf.h
===================================================================
--- sys/mbuf.h	(revision 222695)
+++ sys/mbuf.h	(working copy)
@@ -199,7 +199,9 @@
 #define	M_PROTO6	0x00080000 /* protocol-specific */
 #define	M_PROTO7	0x00100000 /* protocol-specific */
 #define	M_PROTO8	0x00200000 /* protocol-specific */
-#define	M_FLOWID	0x00400000 /* flowid is valid */
+#define	M_FLOWID	0x00400000 /* deprecated: flowid is valid */
+#define	M_HASHTYPEBITS	0x0F000000 /* mask of bits holding flowid hash type */
+
 /*
  * For RELENG_{6,7} steal these flags for limited multiple routing table
  * support. In RELENG_8 and beyond, use just one flag and a tag.
@@ -215,11 +217,43 @@
     (M_PROTO1|M_PROTO2|M_PROTO3|M_PROTO4|M_PROTO5|M_PROTO6|M_PROTO7|M_PROTO8)
 
 /*
+ * Increasingly, network interface cards are able to hash higher-level
+ * protocol fields (such as IP addresses and port numbers) to identify
+ * possible flows that received packets may belong to.  NICs use these hashes
+ * to maintain ordering while load balancing, as well as (in many cases)
+ * provide a stateless affinity model.  When NICs pass up the resulting hash,
+ * the driver will store it in m->m_pkthdr.flowid, and set m_flag bits to
+ * indicate how the hash should be interpreted.
+ *
+ * Most NICs support RSS, which provides ordering and explicit affinity, and
+ * use the hash m_flag bits to indicate what header fields were covered by
+ * the hash.  M_HASHTYPE_OPAQUE can be set by non-RSS cards or configurations
+ * that provide an opaque flow identifier, allowing for ordering and
+ * distribution without explicit affinity.
+ */
+#define	M_HASHTYPE_SHIFT		24
+#define	M_HASHTYPE_NONE			0x0
+#define	M_HASHTYPE_RSS_IPV4		0x1
+#define	M_HASHTYPE_RSS_TCP_IPV4		0x2
+#define	M_HASHTYPE_RSS_IPV6		0x3
+#define	M_HASHTYPE_RSS_TCP_IPV6		0x4
+#define	M_HASHTYPE_RSS_IPV6_EX		0x5
+#define	M_HASHTYPE_RSS_TCP_IPV6_EX	0x6
+#define	M_HASHTYPE_OPAQUE		0xf
+
+#define	M_HASHTYPE_CLEAR(m)	(m)->m_flags &= ~(M_HASHTYPEBITS)
+#define	M_HASHTYPE_GET(m)	(((m)->m_flags & M_HASHTYPEBITS) >> \
+				    M_HASHTYPE_SHIFT)
+#define	M_HASHTYPE_SET(m, v)	(m)->m_flags |= (((v) << M_HASHTYPE_SHIFT) & \
+				     M_HASHTYPEBITS)
+#define	M_HASHTYPE_TEST(m, v)	(M_HASHTYPE_GET(m) == (v))
+
+/*
  * Flags preserved when copying m_pkthdr.
  */
 #define	M_COPYFLAGS \
     (M_PKTHDR|M_EOR|M_RDONLY|M_PROTOFLAGS|M_SKIP_FIREWALL|M_BCAST|M_MCAST|\
-     M_FRAG|M_FIRSTFRAG|M_LASTFRAG|M_VLANTAG|M_PROMISC|M_FIB)
+     M_FRAG|M_FIRSTFRAG|M_LASTFRAG|M_VLANTAG|M_PROMISC|M_FIB|M_HASHTYPEBITS)
 
 /*
  * External buffer types: identify ext_buf type.