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Eric Biggers a5bd029c73 net: add skb_crc32c() 
Add skb_crc32c(), which calculates the CRC32C of a sk_buff.  It will
replace __skb_checksum(), which unnecessarily supports arbitrary
checksums.  Compared to __skb_checksum(), skb_crc32c():

   - Uses the correct type for CRC32C values (u32, not __wsum).

   - Does not require the caller to provide a skb_checksum_ops struct.

   - Is faster because it does not use indirect calls and does not use
     the very slow crc32c_combine().

According to commit 2817a336d4 ("net: skb_checksum: allow custom
update/combine for walking skb") which added __skb_checksum(), the
original motivation for the abstraction layer was to avoid code
duplication for CRC32C and other checksums in the future.  However:

   - No additional checksums showed up after CRC32C.  __skb_checksum()
     is only used with the "regular" net checksum and CRC32C.

   - Indirect calls are expensive.  Commit 2544af0344 ("net: avoid
     indirect calls in L4 checksum calculation") worked around this
     using the INDIRECT_CALL_1 macro. But that only avoided the indirect
     call for the net checksum, and at the cost of an extra branch.

   - The checksums use different types (__wsum and u32), causing casts
     to be needed.

   - It made the checksums of fragments be combined (rather than
     chained) for both checksums, despite this being highly
     counterproductive for CRC32C due to how slow crc32c_combine() is.
     This can clearly be seen in commit 4c2f245496 ("sctp: linearize
     early if it's not GSO") which tried to work around this performance
     bug.  With a dedicated function for each checksum, we can instead
     just use the proper strategy for each checksum.

As shown by the following tables, the new function skb_crc32c() is
faster than __skb_checksum(), with the improvement varying greatly from
5% to 2500% depending on the case.  The largest improvements come from
fragmented packets, mainly due to eliminating the inefficient
crc32c_combine().  But linear packets are improved too, especially
shorter ones, mainly due to eliminating indirect calls.  These
benchmarks were done on AMD Zen 5.  On that CPU, Linux uses IBRS instead
of retpoline; an even greater improvement might be seen with retpoline:

    Linear sk_buffs

        Length in bytes    __skb_checksum cycles    skb_crc32c cycles
        ===============    =====================    =================
                     64                       43                   18
                    256                       94                   77
                   1420                      204                  161
                  16384                     1735                 1642

    Nonlinear sk_buffs (even split between head and one fragment)

        Length in bytes    __skb_checksum cycles    skb_crc32c cycles
        ===============    =====================    =================
                     64                      579                   22
                    256                      829                   77
                   1420                     1506                  194
                  16384                     4365                 1682

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://patch.msgid.link/20250519175012.36581-3-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2025-05-21 15:39:58 -07:00
arch net: phy: fixed_phy: remove irq argument from fixed_phy_add 2025-05-20 18:17:43 -07:00
block block-6.15-20250509 2025-05-09 10:34:50 -07:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-01 15:11:38 -07:00
Documentation netlink: specs: tc: add qdisc dump to TC spec 2025-05-21 12:38:23 -07:00
drivers net: airoha: ppe: Disable packet keepalive 2025-05-20 20:00:51 -07:00
fs Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-15 11:28:30 -07:00
include net: add skb_crc32c() 2025-05-21 15:39:58 -07:00
init init: remove unused CONFIG_CC_CAN_LINK_STATIC 2025-05-12 15:03:46 +09:00
io_uring Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-15 11:28:30 -07:00
ipc treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
kernel Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-15 11:28:30 -07:00
lib pldmfw: Don't require send_package_data or send_component_table to be defined 2025-05-15 12:59:18 +02:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-15 11:28:30 -07:00
net net: add skb_crc32c() 2025-05-21 15:39:58 -07:00
rust rust: clean Rust 1.88.0's clippy::uninlined_format_args lint 2025-05-07 00:11:47 +02:00
samples Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-15 11:28:30 -07:00
scripts Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-15 11:28:30 -07:00
security Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2025-05-01 15:11:38 -07:00
sound ASoC: Fixes for v6.15 2025-05-01 10:22:20 +02:00
tools tools: ynl: add a sample for TC 2025-05-21 12:38:23 -07:00
usr usr/include: openrisc: don't HDRTEST bpf_perf_event.h 2025-05-12 15:03:17 +09:00
virt ARM: 2025-04-08 13:47:55 -07:00
.clang-format clang-format: Update the ForEachMacros list for v6.15-rc1 2025-04-13 11:03:59 +02:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: Retire Ralf Baechle 2024-11-12 15:48:59 +01:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: Create intermediate vmlinux build with relocations preserved 2025-03-17 00:29:50 +09:00
.mailmap soc: fixes for 6.15, part 2 2025-05-14 09:11:05 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: update SLAB ALLOCATOR maintainers 2025-04-17 20:10:06 -07:00
Kbuild drm: ensure drm headers are self-contained and pass kernel-doc 2025-02-12 10:44:43 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS net: phy: mediatek: add driver for built-in 2.5G ethernet PHY on MT7988 2025-05-20 18:11:04 -07:00
Makefile Revert "kbuild, rust: use -fremap-path-prefix to make paths relative" 2025-05-12 15:04:13 +09:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.