Documentation/networking: add checksum-offloads.txt to explain LCO

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/00-INDEX

@@ -44,6 +44,8 @@ can.txt
 	- documentation on CAN protocol family.
 cdc_mbim.txt
 	- 3G/LTE USB modem (Mobile Broadband Interface Model)
+checksum-offloads.txt
+	- Explanation of checksum offloads; LCO, RCO
 cops.txt
 	- info on the COPS LocalTalk Linux driver
 cs89x0.txt

Documentation/networking/checksum-offloads.txt

@@ -0,0 +1,119 @@
+Checksum Offloads in the Linux Networking Stack
+
+
+Introduction
+============
+
+This document describes a set of techniques in the Linux networking stack
+ to take advantage of checksum offload capabilities of various NICs.
+
+The following technologies are described:
+ * TX Checksum Offload
+ * LCO: Local Checksum Offload
+ * RCO: Remote Checksum Offload
+
+Things that should be documented here but aren't yet:
+ * RX Checksum Offload
+ * CHECKSUM_UNNECESSARY conversion
+
+
+TX Checksum Offload
+===================
+
+The interface for offloading a transmit checksum to a device is explained
+ in detail in comments near the top of include/linux/skbuff.h.
+In brief, it allows to request the device fill in a single ones-complement
+ checksum defined by the sk_buff fields skb->csum_start and
+ skb->csum_offset.  The device should compute the 16-bit ones-complement
+ checksum (i.e. the 'IP-style' checksum) from csum_start to the end of the
+ packet, and fill in the result at (csum_start + csum_offset).
+Because csum_offset cannot be negative, this ensures that the previous
+ value of the checksum field is included in the checksum computation, thus
+ it can be used to supply any needed corrections to the checksum (such as
+ the sum of the pseudo-header for UDP or TCP).
+This interface only allows a single checksum to be offloaded.  Where
+ encapsulation is used, the packet may have multiple checksum fields in
+ different header layers, and the rest will have to be handled by another
+ mechanism such as LCO or RCO.
+No offloading of the IP header checksum is performed; it is always done in
+ software.  This is OK because when we build the IP header, we obviously
+ have it in cache, so summing it isn't expensive.  It's also rather short.
+The requirements for GSO are more complicated, because when segmenting an
+ encapsulated packet both the inner and outer checksums may need to be
+ edited or recomputed for each resulting segment.  See the skbuff.h comment
+ (section 'E') for more details.
+
+A driver declares its offload capabilities in netdev->hw_features; see
+ Documentation/networking/netdev-features for more.  Note that a device
+ which only advertises NETIF_F_IP[V6]_CSUM must still obey the csum_start
+ and csum_offset given in the SKB; if it tries to deduce these itself in
+ hardware (as some NICs do) the driver should check that the values in the
+ SKB match those which the hardware will deduce, and if not, fall back to
+ checksumming in software instead (with skb_checksum_help or one of the
+ skb_csum_off_chk* functions as mentioned in include/linux/skbuff.h).  This
+ is a pain, but that's what you get when hardware tries to be clever.
+
+The stack should, for the most part, assume that checksum offload is
+ supported by the underlying device.  The only place that should check is
+ validate_xmit_skb(), and the functions it calls directly or indirectly.
+ That function compares the offload features requested by the SKB (which
+ may include other offloads besides TX Checksum Offload) and, if they are
+ not supported or enabled on the device (determined by netdev->features),
+ performs the corresponding offload in software.  In the case of TX
+ Checksum Offload, that means calling skb_checksum_help(skb).
+
+
+LCO: Local Checksum Offload
+===========================
+
+LCO is a technique for efficiently computing the outer checksum of an
+ encapsulated datagram when the inner checksum is due to be offloaded.
+The ones-complement sum of a correctly checksummed TCP or UDP packet is
+ equal to the sum of the pseudo header, because everything else gets
+ 'cancelled out' by the checksum field.  This is because the sum was
+ complemented before being written to the checksum field.
+More generally, this holds in any case where the 'IP-style' ones complement
+ checksum is used, and thus any checksum that TX Checksum Offload supports.
+That is, if we have set up TX Checksum Offload with a start/offset pair, we
+ know that _after the device has filled in that checksum_, the ones
+ complement sum from csum_start to the end of the packet will be equal to
+ _whatever value we put in the checksum field beforehand_.  This allows us
+ to compute the outer checksum without looking at the payload: we simply
+ stop summing when we get to csum_start, then add the 16-bit word at
+ (csum_start + csum_offset).
+Then, when the true inner checksum is filled in (either by hardware or by
+ skb_checksum_help()), the outer checksum will become correct by virtue of
+ the arithmetic.
+
+LCO is performed by the stack when constructing an outer UDP header for an
+ encapsulation such as VXLAN or GENEVE, in udp_set_csum().  Similarly for
+ the IPv6 equivalents, in udp6_set_csum().
+It is also performed when constructing an IPv4 GRE header, in
+ net/ipv4/ip_gre.c:build_header().  It is *not* currently performed when
+ constructing an IPv6 GRE header; the GRE checksum is computed over the
+ whole packet in net/ipv6/ip6_gre.c:ip6gre_xmit2(), but it should be
+ possible to use LCO here as IPv6 GRE still uses an IP-style checksum.
+All of the LCO implementations use a helper function lco_csum(), in
+ include/linux/skbuff.h.
+
+LCO can safely be used for nested encapsulations; in this case, the outer
+ encapsulation layer will sum over both its own header and the 'middle'
+ header.  This does mean that the 'middle' header will get summed multiple
+ times, but there doesn't seem to be a way to avoid that without incurring
+ bigger costs (e.g. in SKB bloat).
+
+
+RCO: Remote Checksum Offload
+============================
+
+RCO is a technique for eliding the inner checksum of an encapsulated
+ datagram, allowing the outer checksum to be offloaded.  It does, however,
+ involve a change to the encapsulation protocols, which the receiver must
+ also support.  For this reason, it is disabled by default.
+RCO is detailed in the following Internet-Drafts:
+https://tools.ietf.org/html/draft-herbert-remotecsumoffload-00
+https://tools.ietf.org/html/draft-herbert-vxlan-rco-00
+In Linux, RCO is implemented individually in each encapsulation protocol,
+ and most tunnel types have flags controlling its use.  For instance, VXLAN
+ has the flag VXLAN_F_REMCSUM_TX (per struct vxlan_rdst) to indicate that
+ RCO should be used when transmitting to a given remote destination.

include/linux/skbuff.h

@@ -3705,6 +3705,8 @@ static inline unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
 /* Local Checksum Offload.
  * Compute outer checksum based on the assumption that the
  * inner checksum will be offloaded later.
+ * See Documentation/networking/checksum-offloads.txt for
+ * explanation of how this works.
  * Fill in outer checksum adjustment (e.g. with sum of outer
  * pseudo-header) before calling.
  * Also ensure that inner checksum is in linear data area.