[PATCH v3] riscv: Optimize crc32 with Zbc extension
Charlie Jenkins
charlie at rivosinc.com
Wed Mar 13 15:47:00 PDT 2024
On Wed, Mar 13, 2024 at 11:21:39AM +0800, Xiao Wang wrote:
> As suggested by the B-ext spec, the Zbc (carry-less multiplication)
> instructions can be used to accelerate CRC calculations. Currently, the
> crc32 is the most widely used crc function inside kernel, so this patch
> focuses on the optimization of just the crc32 APIs.
>
> Compared with the current table-lookup based optimization, Zbc based
> optimization can also achieve large stride during CRC calculation loop,
> meantime, it avoids the memory access latency of the table-lookup based
> implementation and it reduces memory footprint.
>
> If Zbc feature is not supported in a runtime environment, then the
> table-lookup based implementation would serve as fallback via alternative
> mechanism.
>
> By inspecting the vmlinux built by gcc v12.2.0 with default optimization
> level (-O2), we can see below instruction count change for each 8-byte
> stride in the CRC32 loop:
>
> rv64: crc32_be (54->31), crc32_le (54->13), __crc32c_le (54->13)
> rv32: crc32_be (50->32), crc32_le (50->16), __crc32c_le (50->16)
Even though this loop is optimized, there are a lot of other
instructions being executed else where for these tests. When running the
test-case in QEMU with ZBC enabled, I get these results:
[ 0.353444] crc32: CRC_LE_BITS = 64, CRC_BE BITS = 64
[ 0.353470] crc32: self tests passed, processed 225944 bytes in 2044700 nsec
[ 0.354098] crc32c: CRC_LE_BITS = 64
[ 0.354114] crc32c: self tests passed, processed 112972 bytes in 289000 nsec
[ 0.387204] crc32_combine: 8373 self tests passed
[ 0.419881] crc32c_combine: 8373 self tests passed
Then when running with ZBC disabled I get:
[ 0.351331] crc32: CRC_LE_BITS = 64, CRC_BE BITS = 64
[ 0.351359] crc32: self tests passed, processed 225944 bytes in 567500 nsec
[ 0.352071] crc32c: CRC_LE_BITS = 64
[ 0.352090] crc32c: self tests passed, processed 112972 bytes in 289900 nsec
[ 0.385395] crc32_combine: 8373 self tests passed
[ 0.418180] crc32c_combine: 8373 self tests passed
This is QEMU so it's not a perfect representation of hardware, but being
4 times slower with ZBC seems suspicious. I ran these tests numerous
times and got similar results. Do you know why these tests would perform
4 times better without ZBC?
>
> The compile target CPU is little endian, extra effort is needed for byte
> swapping for the crc32_be API, thus, the instruction count change is not
> as significant as that in the *_le cases.
>
> This patch is tested on QEMU VM with the kernel CRC32 selftest for both
> rv64 and rv32.
>
> Signed-off-by: Xiao Wang <xiao.w.wang at intel.com>
> ---
> v3:
> - Use Zbc to handle also the data head and tail bytes, instead of calling
> fallback function.
> - Misc changes due to the new design.
>
> v2:
> - Fix sparse warnings about type casting. (lkp)
> - Add info about instruction count change in commit log. (Andrew)
> - Use the min() helper from linux/minmax.h. (Andrew)
> - Use "#if __riscv_xlen == 64" macro check to differentiate rv64 and rv32. (Andrew)
> - Line up several macro values by tab. (Andrew)
> - Make poly_qt as "unsigned long" to unify the code for rv64 and rv32. (David)
> - Fix the style of comment wing. (Andrew)
> - Add function wrappers for the asm code for the *_le cases. (Andrew)
> ---
> arch/riscv/Kconfig | 23 ++++
> arch/riscv/lib/Makefile | 1 +
> arch/riscv/lib/crc32.c | 294 ++++++++++++++++++++++++++++++++++++++++
> include/linux/crc32.h | 3 +
> 4 files changed, 321 insertions(+)
> create mode 100644 arch/riscv/lib/crc32.c
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 0bfcfec67ed5..68e1248bbee4 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -610,6 +610,29 @@ config RISCV_ISA_ZBB
>
> If you don't know what to do here, say Y.
>
> +config TOOLCHAIN_HAS_ZBC
> + bool
> + default y
> + depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zbc)
> + depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zbc)
> + depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
> + depends on AS_HAS_OPTION_ARCH
> +
> +config RISCV_ISA_ZBC
> + bool "Zbc extension support for carry-less multiplication instructions"
> + depends on TOOLCHAIN_HAS_ZBC
> + depends on MMU
> + depends on RISCV_ALTERNATIVE
> + default y
> + help
> + Adds support to dynamically detect the presence of the Zbc
> + extension (carry-less multiplication) and enable its usage.
> +
> + The Zbc extension could accelerate CRC (cyclic redundancy check)
> + calculations.
> +
> + If you don't know what to do here, say Y.
> +
> config RISCV_ISA_ZICBOM
> bool "Zicbom extension support for non-coherent DMA operation"
> depends on MMU
> diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
> index bd6e6c1b0497..2b369f51b0a5 100644
> --- a/arch/riscv/lib/Makefile
> +++ b/arch/riscv/lib/Makefile
> @@ -13,6 +13,7 @@ endif
> lib-$(CONFIG_MMU) += uaccess.o
> lib-$(CONFIG_64BIT) += tishift.o
> lib-$(CONFIG_RISCV_ISA_ZICBOZ) += clear_page.o
> +lib-$(CONFIG_RISCV_ISA_ZBC) += crc32.o
>
> obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
> lib-$(CONFIG_RISCV_ISA_V) += xor.o
> diff --git a/arch/riscv/lib/crc32.c b/arch/riscv/lib/crc32.c
> new file mode 100644
> index 000000000000..7f28c4a85a45
> --- /dev/null
> +++ b/arch/riscv/lib/crc32.c
> @@ -0,0 +1,294 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Accelerated CRC32 implementation with Zbc extension.
> + *
> + * Copyright (C) 2024 Intel Corporation
> + */
> +
> +#include <asm/hwcap.h>
> +#include <asm/alternative-macros.h>
> +#include <asm/byteorder.h>
> +
> +#include <linux/types.h>
> +#include <linux/minmax.h>
> +#include <linux/crc32poly.h>
> +#include <linux/crc32.h>
> +#include <linux/byteorder/generic.h>
> +
> +/*
> + * Refer to https://www.corsix.org/content/barrett-reduction-polynomials for
> + * better understanding of how this math works.
> + *
> + * let "+" denotes polynomial add (XOR)
> + * let "-" denotes polynomial sub (XOR)
> + * let "*" denotes polynomial multiplication
> + * let "/" denotes polynomial floor division
> + * let "S" denotes source data, XLEN bit wide
> + * let "P" denotes CRC32 polynomial
> + * let "T" denotes 2^(XLEN+32)
> + * let "QT" denotes quotient of T/P, with the bit for 2^XLEN being implicit
> + *
> + * crc32(S, P)
> + * => S * (2^32) - S * (2^32) / P * P
> + * => lowest 32 bits of: S * (2^32) / P * P
> + * => lowest 32 bits of: S * (2^32) * (T / P) / T * P
> + * => lowest 32 bits of: S * (2^32) * quotient / T * P
> + * => lowest 32 bits of: S * quotient / 2^XLEN * P
> + * => lowest 32 bits of: (clmul_high_part(S, QT) + S) * P
> + * => clmul_low_part(clmul_high_part(S, QT) + S, P)
> + *
> + * In terms of below implementations, the BE case is more intuitive, since the
> + * higher order bit sits at more significant position.
> + */
> +
> +#if __riscv_xlen == 64
> +/* Slide by XLEN bits per iteration */
> +# define STEP_ORDER 3
> +
> +/* Each below polynomial quotient has an implicit bit for 2^XLEN */
> +
> +/* Polynomial quotient of (2^(XLEN+32))/CRC32_POLY, in LE format */
> +# define CRC32_POLY_QT_LE 0x5a72d812fb808b20
> +
> +/* Polynomial quotient of (2^(XLEN+32))/CRC32C_POLY, in LE format */
> +# define CRC32C_POLY_QT_LE 0xa434f61c6f5389f8
> +
> +/* Polynomial quotient of (2^(XLEN+32))/CRC32_POLY, in BE format, it should be
> + * the same as the bit-reversed version of CRC32_POLY_QT_LE
> + */
> +# define CRC32_POLY_QT_BE 0x04d101df481b4e5a
> +
> +static inline u64 crc32_le_prep(u32 crc, unsigned long const *ptr)
> +{
> + return (u64)crc ^ (__force u64)__cpu_to_le64(*ptr);
> +}
> +
> +static inline u32 crc32_le_zbc(unsigned long s, u32 poly, unsigned long poly_qt)
> +{
> + u32 crc;
> +
> + /* We don't have a "clmulrh" insn, so use clmul + slli instead. */
> + asm volatile (".option push\n"
> + ".option arch,+zbc\n"
> + "clmul %0, %1, %2\n"
> + "slli %0, %0, 1\n"
> + "xor %0, %0, %1\n"
> + "clmulr %0, %0, %3\n"
> + "srli %0, %0, 32\n"
> + ".option pop\n"
> + : "=&r" (crc)
> + : "r" (s),
> + "r" (poly_qt),
> + "r" ((u64)poly << 32)
> + :);
> + return crc;
> +}
> +
> +static inline u64 crc32_be_prep(u32 crc, unsigned long const *ptr)
> +{
> + return ((u64)crc << 32) ^ (__force u64)__cpu_to_be64(*ptr);
> +}
> +
> +#elif __riscv_xlen == 32
> +# define STEP_ORDER 2
> +/* Each quotient should match the upper half of its analog in RV64 */
> +# define CRC32_POLY_QT_LE 0xfb808b20
> +# define CRC32C_POLY_QT_LE 0x6f5389f8
> +# define CRC32_POLY_QT_BE 0x04d101df
> +
> +static inline u32 crc32_le_prep(u32 crc, unsigned long const *ptr)
> +{
> + return crc ^ (__force u32)__cpu_to_le32(*ptr);
> +}
> +
> +static inline u32 crc32_le_zbc(unsigned long s, u32 poly, unsigned long poly_qt)
> +{
> + u32 crc;
> +
> + /* We don't have a "clmulrh" insn, so use clmul + slli instead. */
> + asm volatile (".option push\n"
> + ".option arch,+zbc\n"
> + "clmul %0, %1, %2\n"
> + "slli %0, %0, 1\n"
> + "xor %0, %0, %1\n"
> + "clmulr %0, %0, %3\n"
> + ".option pop\n"
> + : "=&r" (crc)
> + : "r" (s),
> + "r" (poly_qt),
> + "r" (poly)
> + :);
> + return crc;
> +}
> +
> +static inline u32 crc32_be_prep(u32 crc, unsigned long const *ptr)
> +{
> + return crc ^ (__force u32)__cpu_to_be32(*ptr);
> +}
> +
> +#else
> +# error "Unexpected __riscv_xlen"
> +#endif
> +
> +static inline u32 crc32_be_zbc(unsigned long s)
> +{
> + u32 crc;
> +
> + asm volatile (".option push\n"
> + ".option arch,+zbc\n"
> + "clmulh %0, %1, %2\n"
> + "xor %0, %0, %1\n"
> + "clmul %0, %0, %3\n"
> + ".option pop\n"
> + : "=&r" (crc)
> + : "r" (s),
> + "r" (CRC32_POLY_QT_BE),
> + "r" (CRC32_POLY_BE)
> + :);
> + return crc;
> +}
> +
> +#define STEP (1 << STEP_ORDER)
> +#define OFFSET_MASK (STEP - 1)
> +
> +typedef u32 (*fallback)(u32 crc, unsigned char const *p, size_t len);
> +
> +static inline u32 crc32_le_unaligned(u32 crc, unsigned char const *p,
> + size_t len, u32 poly,
> + unsigned long poly_qt)
> +{
> + size_t bits = len * 8;
> + unsigned long s = 0;
> + u32 crc_low = 0;
> +
> + for (int i = 0; i < len; i++)
> + s = ((unsigned long)*p++ << (__riscv_xlen - 8)) | (s >> 8);
> +
> + s ^= (unsigned long)crc << (__riscv_xlen - bits);
> + if (__riscv_xlen == 32 || len < sizeof(u32))
> + crc_low = crc >> bits;
> +
> + crc = crc32_le_zbc(s, poly, poly_qt);
> + crc ^= crc_low;
> +
> + return crc;
> +}
> +
> +static inline u32 __pure crc32_le_generic(u32 crc, unsigned char const *p,
> + size_t len, u32 poly,
> + unsigned long poly_qt,
> + fallback crc_fb)
> +{
> + size_t offset, head_len, tail_len;
> + unsigned long const *p_ul;
> + unsigned long s;
> +
> + asm_volatile_goto(ALTERNATIVE("j %l[legacy]", "nop", 0,
This needs to be changed to be asm goto:
4356e9f841f7f ("work around gcc bugs with 'asm goto' with outputs")
> + RISCV_ISA_EXT_ZBC, 1)
> + : : : : legacy);
> +
> + /* Handle the unaligned head. */
> + offset = (unsigned long)p & OFFSET_MASK;
> + if (offset && len) {
If len is 0 nothing in the function seems like it will modify crc. Is there
a reason to not break out immediately if len is 0?
> + head_len = min(STEP - offset, len);
> + crc = crc32_le_unaligned(crc, p, head_len, poly, poly_qt);
> + p += head_len;
> + len -= head_len;
> + }
> +
> + tail_len = len & OFFSET_MASK;
> + len = len >> STEP_ORDER;
> + p_ul = (unsigned long const *)p;
> +
> + for (int i = 0; i < len; i++) {
> + s = crc32_le_prep(crc, p_ul);
> + crc = crc32_le_zbc(s, poly, poly_qt);
> + p_ul++;
> + }
> +
> + /* Handle the tail bytes. */
> + p = (unsigned char const *)p_ul;
> + if (tail_len)
> + crc = crc32_le_unaligned(crc, p, tail_len, poly, poly_qt);
> +
> + return crc;
> +
> +legacy:
> + return crc_fb(crc, p, len);
> +}
> +
> +u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
> +{
> + return crc32_le_generic(crc, p, len, CRC32_POLY_LE, CRC32_POLY_QT_LE,
> + crc32_le_base);
> +}
> +
> +u32 __pure __crc32c_le(u32 crc, unsigned char const *p, size_t len)
> +{
> + return crc32_le_generic(crc, p, len, CRC32C_POLY_LE,
> + CRC32C_POLY_QT_LE, __crc32c_le_base);
> +}
> +
> +static inline u32 crc32_be_unaligned(u32 crc, unsigned char const *p,
> + size_t len)
> +{
> + size_t bits = len * 8;
> + unsigned long s = 0;
> + u32 crc_low = 0;
> +
> + s = 0;
> + for (int i = 0; i < len; i++)
> + s = *p++ | (s << 8);
> +
> + if (__riscv_xlen == 32 || len < sizeof(u32)) {
> + s ^= crc >> (32 - bits);
> + crc_low = crc << bits;
> + } else {
> + s ^= (unsigned long)crc << (bits - 32);
> + }
> +
> + crc = crc32_be_zbc(s);
> + crc ^= crc_low;
> +
> + return crc;
> +}
> +
> +u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
> +{
> + size_t offset, head_len, tail_len;
> + unsigned long const *p_ul;
> + unsigned long s;
> +
> + asm_volatile_goto(ALTERNATIVE("j %l[legacy]", "nop", 0,
Same here
> + RISCV_ISA_EXT_ZBC, 1)
> + : : : : legacy);
> +
> + /* Handle the unaligned head. */
> + offset = (unsigned long)p & OFFSET_MASK;
> + if (offset && len) {
> + head_len = min(STEP - offset, len);
> + crc = crc32_be_unaligned(crc, p, head_len);
> + p += head_len;
> + len -= head_len;
> + }
> +
> + tail_len = len & OFFSET_MASK;
> + len = len >> STEP_ORDER;
> + p_ul = (unsigned long const *)p;
> +
> + for (int i = 0; i < len; i++) {
> + s = crc32_be_prep(crc, p_ul);
> + crc = crc32_be_zbc(s);
> + p_ul++;
> + }
> +
> + /* Handle the tail bytes. */
> + p = (unsigned char const *)p_ul;
> + if (tail_len)
> + crc = crc32_be_unaligned(crc, p, tail_len);
> +
> + return crc;
> +
> +legacy:
> + return crc32_be_base(crc, p, len);
> +}
> diff --git a/include/linux/crc32.h b/include/linux/crc32.h
> index 9e8a032c1788..87f788c0d607 100644
> --- a/include/linux/crc32.h
> +++ b/include/linux/crc32.h
> @@ -9,7 +9,9 @@
> #include <linux/bitrev.h>
>
> u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len);
> +u32 __pure crc32_le_base(u32 crc, unsigned char const *p, size_t len);
> u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len);
> +u32 __pure crc32_be_base(u32 crc, unsigned char const *p, size_t len);
>
> /**
> * crc32_le_combine - Combine two crc32 check values into one. For two
> @@ -37,6 +39,7 @@ static inline u32 crc32_le_combine(u32 crc1, u32 crc2, size_t len2)
> }
>
> u32 __pure __crc32c_le(u32 crc, unsigned char const *p, size_t len);
> +u32 __pure __crc32c_le_base(u32 crc, unsigned char const *p, size_t len);
>
> /**
> * __crc32c_le_combine - Combine two crc32c check values into one. For two
> --
> 2.25.1
>
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