forked from Minki/linux
43c26a1a45
skb_csum_hwoffload_help() uses netdev features and skb->csum_not_inet to
determine if skb needs software computation of Internet Checksum or crc32c
(or nothing, if this computation can be done by the hardware). Use it in
place of skb_checksum_help() in validate_xmit_skb() to avoid corruption
of non-GSO SCTP packets having skb->ip_summed equal to CHECKSUM_PARTIAL.
While at it, remove references to skb_csum_off_chk* functions, since they
are not present anymore in Linux _ see commit cf53b1da73 ("Revert
"net: Add driver helper functions to determine checksum offloadability"").
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
123 lines
6.3 KiB
Plaintext
123 lines
6.3 KiB
Plaintext
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.
|
|
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 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.
|