From: Jakub Kicinski <kuba@kernel.org>
To: davem@davemloft.net
Cc: netdev@vger.kernel.org, edumazet@google.com, pabeni@redhat.com,
andrew+netdev@lunn.ch, horms@kernel.org, corbet@lwn.net,
vladimir.oltean@nxp.com, willemb@google.com,
sdf.kernel@gmail.com, ecree.xilinx@gmail.com,
jesse.brandeburg@intel.com, linux-doc@vger.kernel.org,
Jakub Kicinski <kuba@kernel.org>
Subject: [PATCH net-next 07/10] docs: net: fix minor issues with checksum offloads
Date: Tue, 26 May 2026 09:01:48 -0700 [thread overview]
Message-ID: <20260526160151.2793354-8-kuba@kernel.org> (raw)
In-Reply-To: <20260526160151.2793354-1-kuba@kernel.org>
Update the checksum offload documentation to match current code:
- SCTP CRC32c offload requires NETIF_F_SCTP_CRC, not ordinary IP
checksum offload
- NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM are restricted legacy
features; new devices should use NETIF_F_HW_CSUM
- GRE LCO is handled by the shared gre_build_header() helper used by
both IPv4 and IPv6 GRE
- VXLAN_F_REMCSUM_TX is a VXLAN configuration flag, not a field of
struct vxlan_rdst
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
.../networking/checksum-offloads.rst | 36 +++++++++----------
1 file changed, 17 insertions(+), 19 deletions(-)
diff --git a/Documentation/networking/checksum-offloads.rst b/Documentation/networking/checksum-offloads.rst
index 69b23cf6879e..907aed9f3a3b 100644
--- a/Documentation/networking/checksum-offloads.rst
+++ b/Documentation/networking/checksum-offloads.rst
@@ -45,9 +45,11 @@ 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.
+SCTP CRC32c can also be offloaded using this interface, by means of filling
+skb->csum_start and skb->csum_offset as described above, setting
+skb->csum_not_inet, and advertising NETIF_F_SCTP_CRC. Drivers must not treat
+ordinary IP checksum offload as SCTP CRC32c support. See the 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
@@ -59,14 +61,12 @@ 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.rst 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).
+Documentation/networking/netdev-features.rst for more. NETIF_F_IP_CSUM and
+NETIF_F_IPV6_CSUM are restricted legacy features and are being deprecated in
+favor of NETIF_F_HW_CSUM. New devices should use NETIF_F_HW_CSUM to advertise
+generic checksum offload. The skb_csum_hwoffload_help() helper can resolve
+CHECKSUM_PARTIAL according to the device's advertised checksum capabilities,
+falling back to software when needed.
The stack should, for the most part, assume that checksum offload is supported
by the underlying device. The only place that should check is
@@ -108,11 +108,9 @@ 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.
+It is also performed when constructing GRE headers with the shared
+gre_build_header() helper in include/net/gre.h, which is used by both IPv4 and
+IPv6 GRE.
All of the LCO implementations use a helper function lco_csum(), in
include/linux/skbuff.h.
@@ -138,6 +136,6 @@ For this reason, it is disabled by default.
* 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.
+most tunnel types have flags controlling its use. For instance, VXLAN has the
+configuration flag VXLAN_F_REMCSUM_TX to indicate that RCO should be used when
+transmitting.
--
2.54.0
next prev parent reply other threads:[~2026-05-26 16:02 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-26 16:01 [PATCH net-next 00/10] docs: net: updates for old and cobwebbed docs Jakub Kicinski
2026-05-26 16:01 ` [PATCH net-next 01/10] docs: net: netdevices: small fixes and clarifications Jakub Kicinski
2026-05-26 22:12 ` Stanislav Fomichev
2026-05-26 16:01 ` [PATCH net-next 02/10] docs: net: fix minor issues with driver guide Jakub Kicinski
2026-05-26 16:01 ` [PATCH net-next 03/10] docs: net: statistics: fix kernel-internal stats list Jakub Kicinski
2026-05-26 16:01 ` [PATCH net-next 04/10] docs: net: update devmem code examples Jakub Kicinski
2026-05-26 22:17 ` Stanislav Fomichev
2026-05-26 16:01 ` [PATCH net-next 05/10] docs: net: fix minor issues with the NAPI guide Jakub Kicinski
2026-05-26 16:01 ` [PATCH net-next 06/10] docs: net: refresh netdev feature guidance Jakub Kicinski
2026-05-26 18:41 ` Maxime Chevallier
2026-05-26 22:35 ` Jakub Kicinski
2026-05-27 7:53 ` Maxime Chevallier
2026-05-26 16:01 ` Jakub Kicinski [this message]
2026-05-26 16:01 ` [PATCH net-next 08/10] docs: net: add Rx notes to the checksum guide Jakub Kicinski
2026-05-26 18:56 ` Willem de Bruijn
2026-05-26 16:01 ` [PATCH net-next 09/10] docs: net: render the checksum comment in checksum-offloads.rst Jakub Kicinski
2026-05-26 18:56 ` Willem de Bruijn
2026-05-26 16:01 ` [PATCH net-next 10/10] docs: net: fix minor issues with segmentation offloads Jakub Kicinski
2026-05-26 18:48 ` [PATCH net-next 00/10] docs: net: updates for old and cobwebbed docs Randy Dunlap
2026-05-26 22:37 ` Jakub Kicinski
2026-05-26 22:40 ` Jakub Kicinski
2026-05-27 0:51 ` Randy Dunlap
2026-05-27 1:15 ` Jakub Kicinski
2026-05-27 2:39 ` Randy Dunlap
2026-05-27 14:48 ` Jakub Kicinski
2026-05-28 22:10 ` patchwork-bot+netdevbpf
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20260526160151.2793354-8-kuba@kernel.org \
--to=kuba@kernel.org \
--cc=andrew+netdev@lunn.ch \
--cc=corbet@lwn.net \
--cc=davem@davemloft.net \
--cc=ecree.xilinx@gmail.com \
--cc=edumazet@google.com \
--cc=horms@kernel.org \
--cc=jesse.brandeburg@intel.com \
--cc=linux-doc@vger.kernel.org \
--cc=netdev@vger.kernel.org \
--cc=pabeni@redhat.com \
--cc=sdf.kernel@gmail.com \
--cc=vladimir.oltean@nxp.com \
--cc=willemb@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.