[PATCH 1/2] lib/crc: simplify the kconfig options for CRC implementations

Eric Biggers
2025-01-23T21:33:06Z

From: Eric Biggers <ebiggers@google.com>

Make the following simplifications to the kconfig options for choosing
CRC implementations for CRC32 and CRC_T10DIF:

1. Make the option to disable the arch-optimized code be visible only
when CONFIG_EXPERT=y.
2. Make a single option control the inclusion of the arch-optimized code
for all enabled CRC variants.
3. Make CRC32_SARWATE (a.k.a. slice-by-1 or byte-by-byte) be the only
generic CRC32 implementation.

The result is there is now just one option, CRC_OPTIMIZATIONS, which is
default y and can be disabled only when CONFIG_EXPERT=y.

Rationale:

1. Enabling the arch-optimized code is nearly always the right choice.
However, people trying to build the tiniest kernel possible would
find some use in disabling it. Anything we add to CRC32 is de facto
unconditional, given that CRC32 gets selected by something in nearly
all kernels. And unfortunately enabling the arch CRC code does not
eliminate the need to build the generic CRC code into the kernel too,
due to CPU feature dependencies. The size of the arch CRC code will
also increase slightly over time as more CRC variants get added and
more implementations targeting different instruction set extensions
get added. Thus, it seems worthwhile to still provide an option to
disable it, but it should be considered an expert-level tweak.

2. Considering the use case described in (1), there doesn't seem to be
sufficient value in making the arch-optimized CRC code be
independently configurable for different CRC variants. Note also
that multiple variants were already grouped together, e.g.
CONFIG_CRC32 actually enables three different variants of CRC32.

3. The bit-by-bit implementation is uselessly slow, whereas slice-by-n
for n=4 and n=8 use tables that are inconveniently large: 4096 bytes
and 8192 bytes respectively, compared to 1024 bytes for n=1. Higher
n gives higher instruction-level parallelism, so higher n easily wins
on traditional microbenchmarks on most CPUs. However, the larger
tables, which are accessed randomly, can be harmful in real-world
situations where the dcache may be cold or useful data may need be
evicted from the dcache. Meanwhile, today most architectures have
much faster CRC32 implementations using dedicated CRC32 instructions
or carryless multiplication instructions anyway, which make the
generic code obsolete in most cases especially on long messages.

Another reason for going with n=1 is that this is already what is
used by all the other CRC variants in the kernel. CRC32 was unique
in having support for larger tables. But as per the above this can
be considered an outdated optimization.

The standardization on slice-by-1 a.k.a. CRC32_SARWATE makes much of
the code in lib/crc32.c unused. A later patch will clean that up.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
lib/Kconfig | 116 +++++++---------------------------------------------
1 file changed, 14 insertions(+), 102 deletions(-)

diff --git a/lib/Kconfig b/lib/Kconfig
index a78d22c6507f..e08b26e8e03f 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -162,38 +162,13 @@ config CRC_T10DIF
SCSI data integrity subsystem.

config ARCH_HAS_CRC_T10DIF
bool

-choice
- prompt "CRC-T10DIF implementation"
- depends on CRC_T10DIF
- default CRC_T10DIF_IMPL_ARCH if ARCH_HAS_CRC_T10DIF
- default CRC_T10DIF_IMPL_GENERIC if !ARCH_HAS_CRC_T10DIF
- help
- This option allows you to override the default choice of CRC-T10DIF
- implementation.
-
-config CRC_T10DIF_IMPL_ARCH
- bool "Architecture-optimized" if ARCH_HAS_CRC_T10DIF
- help
- Use the optimized implementation of CRC-T10DIF for the selected
- architecture. It is recommended to keep this enabled, as it can
- greatly improve CRC-T10DIF performance.
-
-config CRC_T10DIF_IMPL_GENERIC
- bool "Generic implementation"
- help
- Use the generic table-based implementation of CRC-T10DIF. Selecting
- this will reduce code size slightly but can greatly reduce CRC-T10DIF
- performance.
-
-endchoice
-
config CRC_T10DIF_ARCH
tristate
- default CRC_T10DIF if CRC_T10DIF_IMPL_ARCH
+ default CRC_T10DIF if ARCH_HAS_CRC_T10DIF && CRC_OPTIMIZATIONS

config CRC64_ROCKSOFT
tristate "CRC calculation for the Rocksoft model CRC64"
select CRC64
select CRYPTO
@@ -212,100 +187,26 @@ config CRC_ITU_T

config CRC32
tristate "CRC32/CRC32c functions"
default y
select BITREVERSE
+ select CRC32_SARWATE
help
This option is provided for the case where no in-kernel-tree
modules require CRC32/CRC32c functions, but a module built outside
the kernel tree does. Such modules that use library CRC32/CRC32c
functions require M here.

config ARCH_HAS_CRC32
bool

-choice
- prompt "CRC32 implementation"
- depends on CRC32
- default CRC32_IMPL_ARCH_PLUS_SLICEBY8 if ARCH_HAS_CRC32
- default CRC32_IMPL_SLICEBY8 if !ARCH_HAS_CRC32
- help
- This option allows you to override the default choice of CRC32
- implementation. Choose the default unless you know that you need one
- of the others.
-
-config CRC32_IMPL_ARCH_PLUS_SLICEBY8
- bool "Arch-optimized, with fallback to slice-by-8" if ARCH_HAS_CRC32
- help
- Use architecture-optimized implementation of CRC32. Fall back to
- slice-by-8 in cases where the arch-optimized implementation cannot be
- used, e.g. if the CPU lacks support for the needed instructions.
-
- This is the default when an arch-optimized implementation exists.
-
-config CRC32_IMPL_ARCH_PLUS_SLICEBY1
- bool "Arch-optimized, with fallback to slice-by-1" if ARCH_HAS_CRC32
- help
- Use architecture-optimized implementation of CRC32, but fall back to
- slice-by-1 instead of slice-by-8 in order to reduce the binary size.
-
-config CRC32_IMPL_SLICEBY8
- bool "Slice by 8 bytes"
- help
- Calculate checksum 8 bytes at a time with a clever slicing algorithm.
- This is much slower than the architecture-optimized implementation of
- CRC32 (if the selected arch has one), but it is portable and is the
- fastest implementation when no arch-optimized implementation is
- available. It uses an 8KiB lookup table. Most modern processors have
- enough cache to hold this table without thrashing the cache.
-
-config CRC32_IMPL_SLICEBY4
- bool "Slice by 4 bytes"
- help
- Calculate checksum 4 bytes at a time with a clever slicing algorithm.
- This is a bit slower than slice by 8, but has a smaller 4KiB lookup
- table.
-
- Only choose this option if you know what you are doing.
-
-config CRC32_IMPL_SLICEBY1
- bool "Slice by 1 byte (Sarwate's algorithm)"
- help
- Calculate checksum a byte at a time using Sarwate's algorithm. This
- is not particularly fast, but has a small 1KiB lookup table.
-
- Only choose this option if you know what you are doing.
-
-config CRC32_IMPL_BIT
- bool "Classic Algorithm (one bit at a time)"
- help
- Calculate checksum one bit at a time. This is VERY slow, but has
- no lookup table. This is provided as a debugging option.
-
- Only choose this option if you are debugging crc32.
-
-endchoice
-
config CRC32_ARCH
tristate
- default CRC32 if CRC32_IMPL_ARCH_PLUS_SLICEBY8 || CRC32_IMPL_ARCH_PLUS_SLICEBY1
-
-config CRC32_SLICEBY8
- bool
- default y if CRC32_IMPL_SLICEBY8 || CRC32_IMPL_ARCH_PLUS_SLICEBY8
-
-config CRC32_SLICEBY4
- bool
- default y if CRC32_IMPL_SLICEBY4
+ default CRC32 if ARCH_HAS_CRC32 && CRC_OPTIMIZATIONS

config CRC32_SARWATE
bool
- default y if CRC32_IMPL_SLICEBY1 || CRC32_IMPL_ARCH_PLUS_SLICEBY1
-
-config CRC32_BIT
- bool
- default y if CRC32_IMPL_BIT

config CRC64
tristate "CRC64 functions"
help
This option is provided for the case where no in-kernel-tree
@@ -341,10 +242,21 @@ config CRC8
help
This option provides CRC8 function. Drivers may select this
when they need to do cyclic redundancy check according CRC8
algorithm. Module will be called crc8.

+config CRC_OPTIMIZATIONS
+ bool "Enable optimized CRC implementations" if EXPERT
+ default y
+ help
+ Disabling this option reduces code size slightly by disabling the
+ architecture-optimized implementations of any CRC variants that are
+ enabled. CRC checksumming performance may get much slower.
+
+ Keep this enabled unless you're really trying to minimize the size of
+ the kernel.
+
config XXHASH
tristate

config AUDIT_GENERIC
bool
--
2.48.1