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| author | Jakub Kicinski <kuba@kernel.org> | 2026-05-26 09:01:48 -0700 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2026-05-28 14:37:47 -0700 |
| commit | f94874c38886b4e1adbd3233a313f320966d30ff (patch) | |
| tree | b81cd8655478a71a9b684c54e30a09ceb9b236e1 | |
| parent | f7d7d7e2a53c320503823748e8a28ddd9b01ef70 (diff) | |
| download | linux-next-history-f94874c38886b4e1adbd3233a313f320966d30ff.tar.gz | |
docs: net: fix minor issues with checksum offloads
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
Link: https://patch.msgid.link/20260526160151.2793354-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
| -rw-r--r-- | Documentation/networking/checksum-offloads.rst | 36 |
1 files changed, 17 insertions, 19 deletions
diff --git a/Documentation/networking/checksum-offloads.rst b/Documentation/networking/checksum-offloads.rst index 69b23cf6879e7..907aed9f3a3b8 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 @@ RCO is detailed in the following Internet-Drafts: * 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. |