net: add NSH header structures and helpers

NSH (Network Service Header)[1] is a new protocol for service
function chaining, it can be handled as a L3 protocol like
IPv4 and IPv6, Eth + NSH + Inner packet or VxLAN-gpe + NSH +
Inner packet are two typical use cases.

This patch adds NSH header structures and helpers for NSH GSO
support and Open vSwitch NSH support.

[1] https://datatracker.ietf.org/doc/draft-ietf-sfc-nsh/

[Jiri: added nsh_hdr() helper and renamed the header struct to "struct
nshhdr" to match the usual pattern. Removed packet type defines, these are
now shared with VXLAN-GPE.]
Signed-off-by: NYi Yang <yi.y.yang@intel.com>
Signed-off-by: NJiri Benc <jbenc@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/nsh.h

+#ifndef __NET_NSH_H
+#define __NET_NSH_H 1
+
+#include <linux/skbuff.h>
+
+/*
+ * Network Service Header:
+ *  0                   1                   2                   3
+ *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |Ver|O|U|    TTL    |   Length  |U|U|U|U|MD Type| Next Protocol |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |          Service Path Identifier (SPI)        | Service Index |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |                                                               |
+ * ~               Mandatory/Optional Context Headers              ~
+ * |                                                               |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *
+ * Version: The version field is used to ensure backward compatibility
+ * going forward with future NSH specification updates.  It MUST be set
+ * to 0x0 by the sender, in this first revision of NSH.  Given the
+ * widespread implementation of existing hardware that uses the first
+ * nibble after an MPLS label stack for ECMP decision processing, this
+ * document reserves version 01b and this value MUST NOT be used in
+ * future versions of the protocol.  Please see [RFC7325] for further
+ * discussion of MPLS-related forwarding requirements.
+ *
+ * O bit: Setting this bit indicates an Operations, Administration, and
+ * Maintenance (OAM) packet.  The actual format and processing of SFC
+ * OAM packets is outside the scope of this specification (see for
+ * example [I-D.ietf-sfc-oam-framework] for one approach).
+ *
+ * The O bit MUST be set for OAM packets and MUST NOT be set for non-OAM
+ * packets.  The O bit MUST NOT be modified along the SFP.
+ *
+ * SF/SFF/SFC Proxy/Classifier implementations that do not support SFC
+ * OAM procedures SHOULD discard packets with O bit set, but MAY support
+ * a configurable parameter to enable forwarding received SFC OAM
+ * packets unmodified to the next element in the chain.  Forwarding OAM
+ * packets unmodified by SFC elements that do not support SFC OAM
+ * procedures may be acceptable for a subset of OAM functions, but can
+ * result in unexpected outcomes for others, thus it is recommended to
+ * analyze the impact of forwarding an OAM packet for all OAM functions
+ * prior to enabling this behavior.  The configurable parameter MUST be
+ * disabled by default.
+ *
+ * TTL: Indicates the maximum SFF hops for an SFP.  This field is used
+ * for service plane loop detection.  The initial TTL value SHOULD be
+ * configurable via the control plane; the configured initial value can
+ * be specific to one or more SFPs.  If no initial value is explicitly
+ * provided, the default initial TTL value of 63 MUST be used.  Each SFF
+ * involved in forwarding an NSH packet MUST decrement the TTL value by
+ * 1 prior to NSH forwarding lookup.  Decrementing by 1 from an incoming
+ * value of 0 shall result in a TTL value of 63.  The packet MUST NOT be
+ * forwarded if TTL is, after decrement, 0.
+ *
+ * All other flag fields, marked U, are unassigned and available for
+ * future use, see Section 11.2.1.  Unassigned bits MUST be set to zero
+ * upon origination, and MUST be ignored and preserved unmodified by
+ * other NSH supporting elements.  Elements which do not understand the
+ * meaning of any of these bits MUST NOT modify their actions based on
+ * those unknown bits.
+ *
+ * Length: The total length, in 4-byte words, of NSH including the Base
+ * Header, the Service Path Header, the Fixed Length Context Header or
+ * Variable Length Context Header(s).  The length MUST be 0x6 for MD
+ * Type equal to 0x1, and MUST be 0x2 or greater for MD Type equal to
+ * 0x2.  The length of the NSH header MUST be an integer multiple of 4
+ * bytes, thus variable length metadata is always padded out to a
+ * multiple of 4 bytes.
+ *
+ * MD Type: Indicates the format of NSH beyond the mandatory Base Header
+ * and the Service Path Header.  MD Type defines the format of the
+ * metadata being carried.
+ *
+ * 0x0 - This is a reserved value.  Implementations SHOULD silently
+ * discard packets with MD Type 0x0.
+ *
+ * 0x1 - This indicates that the format of the header includes a fixed
+ * length Context Header (see Figure 4 below).
+ *
+ * 0x2 - This does not mandate any headers beyond the Base Header and
+ * Service Path Header, but may contain optional variable length Context
+ * Header(s).  The semantics of the variable length Context Header(s)
+ * are not defined in this document.  The format of the optional
+ * variable length Context Headers is provided in Section 2.5.1.
+ *
+ * 0xF - This value is reserved for experimentation and testing, as per
+ * [RFC3692].  Implementations not explicitly configured to be part of
+ * an experiment SHOULD silently discard packets with MD Type 0xF.
+ *
+ * Next Protocol: indicates the protocol type of the encapsulated data.
+ * NSH does not alter the inner payload, and the semantics on the inner
+ * protocol remain unchanged due to NSH service function chaining.
+ * Please see the IANA Considerations section below, Section 11.2.5.
+ *
+ * This document defines the following Next Protocol values:
+ *
+ * 0x1: IPv4
+ * 0x2: IPv6
+ * 0x3: Ethernet
+ * 0x4: NSH
+ * 0x5: MPLS
+ * 0xFE: Experiment 1
+ * 0xFF: Experiment 2
+ *
+ * Packets with Next Protocol values not supported SHOULD be silently
+ * dropped by default, although an implementation MAY provide a
+ * configuration parameter to forward them.  Additionally, an
+ * implementation not explicitly configured for a specific experiment
+ * [RFC3692] SHOULD silently drop packets with Next Protocol values 0xFE
+ * and 0xFF.
+ *
+ * Service Path Identifier (SPI): Identifies a service path.
+ * Participating nodes MUST use this identifier for Service Function
+ * Path selection.  The initial classifier MUST set the appropriate SPI
+ * for a given classification result.
+ *
+ * Service Index (SI): Provides location within the SFP.  The initial
+ * classifier for a given SFP SHOULD set the SI to 255, however the
+ * control plane MAY configure the initial value of SI as appropriate
+ * (i.e., taking into account the length of the service function path).
+ * The Service Index MUST be decremented by a value of 1 by Service
+ * Functions or by SFC Proxy nodes after performing required services
+ * and the new decremented SI value MUST be used in the egress packet's
+ * NSH.  The initial Classifier MUST send the packet to the first SFF in
+ * the identified SFP for forwarding along an SFP.  If re-classification
+ * occurs, and that re-classification results in a new SPI, the
+ * (re)classifier is, in effect, the initial classifier for the
+ * resultant SPI.
+ *
+ * The SI is used in conjunction the with Service Path Identifier for
+ * Service Function Path Selection and for determining the next SFF/SF
+ * in the path.  The SI is also valuable when troubleshooting or
+ * reporting service paths.  Additionally, while the TTL field is the
+ * main mechanism for service plane loop detection, the SI can also be
+ * used for detecting service plane loops.
+ *
+ * When the Base Header specifies MD Type = 0x1, a Fixed Length Context
+ * Header (16-bytes) MUST be present immediately following the Service
+ * Path Header. The value of a Fixed Length Context
+ * Header that carries no metadata MUST be set to zero.
+ *
+ * When the base header specifies MD Type = 0x2, zero or more Variable
+ * Length Context Headers MAY be added, immediately following the
+ * Service Path Header (see Figure 5).  Therefore, Length = 0x2,
+ * indicates that only the Base Header followed by the Service Path
+ * Header are present.  The optional Variable Length Context Headers
+ * MUST be of an integer number of 4-bytes.  The base header Length
+ * field MUST be used to determine the offset to locate the original
+ * packet or frame for SFC nodes that require access to that
+ * information.
+ *
+ * The format of the optional variable length Context Headers
+ *
+ *  0                   1                   2                   3
+ *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |          Metadata Class       |      Type     |U|    Length   |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |                      Variable Metadata                        |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *
+ * Metadata Class (MD Class): Defines the scope of the 'Type' field to
+ * provide a hierarchical namespace.  The IANA Considerations
+ * Section 11.2.4 defines how the MD Class values can be allocated to
+ * standards bodies, vendors, and others.
+ *
+ * Type: Indicates the explicit type of metadata being carried.  The
+ * definition of the Type is the responsibility of the MD Class owner.
+ *
+ * Unassigned bit: One unassigned bit is available for future use. This
+ * bit MUST NOT be set, and MUST be ignored on receipt.
+ *
+ * Length: Indicates the length of the variable metadata, in bytes.  In
+ * case the metadata length is not an integer number of 4-byte words,
+ * the sender MUST add pad bytes immediately following the last metadata
+ * byte to extend the metadata to an integer number of 4-byte words.
+ * The receiver MUST round up the length field to the nearest 4-byte
+ * word boundary, to locate and process the next field in the packet.
+ * The receiver MUST access only those bytes in the metadata indicated
+ * by the length field (i.e., actual number of bytes) and MUST ignore
+ * the remaining bytes up to the nearest 4-byte word boundary.  The
+ * Length may be 0 or greater.
+ *
+ * A value of 0 denotes a Context Header without a Variable Metadata
+ * field.
+ *
+ * [0] https://datatracker.ietf.org/doc/draft-ietf-sfc-nsh/
+ */
+
+/**
+ * struct nsh_md1_ctx - Keeps track of NSH context data
+ * @nshc<1-4>: NSH Contexts.
+ */
+struct nsh_md1_ctx {
+	__be32 context[4];
+};
+
+struct nsh_md2_tlv {
+	__be16 md_class;
+	u8 type;
+	u8 length;
+	u8 md_value[];
+};
+
+struct nshhdr {
+	__be16 ver_flags_ttl_len;
+	u8 mdtype;
+	u8 np;
+	__be32 path_hdr;
+	union {
+	    struct nsh_md1_ctx md1;
+	    struct nsh_md2_tlv md2;
+	};
+};
+
+/* Masking NSH header fields. */
+#define NSH_VER_MASK       0xc000
+#define NSH_VER_SHIFT      14
+#define NSH_FLAGS_MASK     0x3000
+#define NSH_FLAGS_SHIFT    12
+#define NSH_TTL_MASK       0x0fc0
+#define NSH_TTL_SHIFT      6
+#define NSH_LEN_MASK       0x003f
+#define NSH_LEN_SHIFT      0
+
+#define NSH_MDTYPE_MASK    0x0f
+#define NSH_MDTYPE_SHIFT   0
+
+#define NSH_SPI_MASK       0xffffff00
+#define NSH_SPI_SHIFT      8
+#define NSH_SI_MASK        0x000000ff
+#define NSH_SI_SHIFT       0
+
+/* MD Type Registry. */
+#define NSH_M_TYPE1     0x01
+#define NSH_M_TYPE2     0x02
+#define NSH_M_EXP1      0xFE
+#define NSH_M_EXP2      0xFF
+
+/* NSH Base Header Length */
+#define NSH_BASE_HDR_LEN  8
+
+/* NSH MD Type 1 header Length. */
+#define NSH_M_TYPE1_LEN   24
+
+/* NSH header maximum Length. */
+#define NSH_HDR_MAX_LEN 256
+
+/* NSH context headers maximum Length. */
+#define NSH_CTX_HDRS_MAX_LEN 248
+
+static inline struct nshhdr *nsh_hdr(struct sk_buff *skb)
+{
+	return (struct nshhdr *)skb_network_header(skb);
+}
+
+static inline u16 nsh_hdr_len(const struct nshhdr *nsh)
+{
+	return ((ntohs(nsh->ver_flags_ttl_len) & NSH_LEN_MASK)
+		>> NSH_LEN_SHIFT) << 2;
+}
+
+static inline u8 nsh_get_ver(const struct nshhdr *nsh)
+{
+	return (ntohs(nsh->ver_flags_ttl_len) & NSH_VER_MASK)
+		>> NSH_VER_SHIFT;
+}
+
+static inline u8 nsh_get_flags(const struct nshhdr *nsh)
+{
+	return (ntohs(nsh->ver_flags_ttl_len) & NSH_FLAGS_MASK)
+		>> NSH_FLAGS_SHIFT;
+}
+
+static inline u8 nsh_get_ttl(const struct nshhdr *nsh)
+{
+	return (ntohs(nsh->ver_flags_ttl_len) & NSH_TTL_MASK)
+		>> NSH_TTL_SHIFT;
+}
+
+static inline void __nsh_set_xflag(struct nshhdr *nsh, u16 xflag, u16 xmask)
+{
+	nsh->ver_flags_ttl_len
+		= (nsh->ver_flags_ttl_len & ~htons(xmask)) | htons(xflag);
+}
+
+static inline void nsh_set_flags_and_ttl(struct nshhdr *nsh, u8 flags, u8 ttl)
+{
+	__nsh_set_xflag(nsh, ((flags << NSH_FLAGS_SHIFT) & NSH_FLAGS_MASK) |
+			     ((ttl << NSH_TTL_SHIFT) & NSH_TTL_MASK),
+			NSH_FLAGS_MASK | NSH_TTL_MASK);
+}
+
+static inline void nsh_set_flags_ttl_len(struct nshhdr *nsh, u8 flags,
+					 u8 ttl, u8 len)
+{
+	len = len >> 2;
+	__nsh_set_xflag(nsh, ((flags << NSH_FLAGS_SHIFT) & NSH_FLAGS_MASK) |
+			     ((ttl << NSH_TTL_SHIFT) & NSH_TTL_MASK) |
+			     ((len << NSH_LEN_SHIFT) & NSH_LEN_MASK),
+			NSH_FLAGS_MASK | NSH_TTL_MASK | NSH_LEN_MASK);
+}
+
+#endif /* __NET_NSH_H */