mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-15 15:04:27 +08:00
b83eb68cb9
The titles do not look very nice in the table of contents generated by Sphinx. I also think it is obvious that the documents are describing offloads in the Linux Networking Stack. Signed-off-by: Otto Sabart <ottosabart@seberm.com> Acked-by: David S. Miller <davem@davemloft.net> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
144 lines
6.3 KiB
ReStructuredText
144 lines
6.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
=================
|
|
Checksum Offloads
|
|
=================
|
|
|
|
|
|
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.
|
|
|
|
CRC32c can also be offloaded using this interface, by means of filling
|
|
skb->csum_start and skb->csum_offset as described above, and setting
|
|
skb->csum_not_inet: see skbuff.h comment (section 'D') for more details.
|
|
|
|
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.txt 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_csum_hwoffload_help() or one of the
|
|
skb_checksum_help() / skb_crc32c_csum_help functions, as mentioned in
|
|
include/linux/skbuff.h).
|
|
|
|
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_csum_hwoffload_help(skb, features).
|
|
|
|
|
|
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 complement of 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 the complement of
|
|
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 complement of 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.
|