Re: [PATCH v2] lib/crc: arm64: add NEON accelerated CRC64-NVMe implementation

Re: [PATCH v2] lib/crc: arm64: add NEON accelerated CRC64-NVMe implementation

[Eric Biggers]

[+Cc linux-arm-kernel@lists.infradead.org]

Thanks!  This is almost ready.  Just a few more comments:

On Fri, Mar 27, 2026 at 06:02:11AM +0000, Demian Shulhan wrote:
> - Safely falls back to the generic implementation on Big-Endian systems.

Drop the above bullet point.  This patch doesn't explicitly exclude big
endian.  Which is correct: Linux arm64 is little-endian-only now.

> +	/*
> +	 * Reduce the 128-bit value to 64 bits.
> +	 * By multiplying the high 64 bits by x^127 mod G (fold_consts_val[1])
> +	 * and XORing the result with the low 64 bits.
> +	 */

That is not what this code does.  How about something like:

	/* Multiply the 128-bit value by x^64 and reduce it back to 128 bits. */

Granted, that doesn't do a good job explaining it either.  However, a
full explanation of this stuff, like the one in the comments in
lib/crc/x86/crc-pclmul-template.S, would be much longer.

I suggest we leave the full explanation for when a similar template is
written for arm64.  For now brief comments or even no comments are fine.

Just if any comments are included they really ought to be correct, as
otherwise they are worse than no comments.

> +			scoped_ksimd() crc = crc64_nvme_arm64_c(crc, p, chunk);

clang-format doesn't know about scoped_ksimd(), so I suggest overriding
the formatting in this particular case:

	scoped_ksimd()
		crc = crc64_nvme_arm64_c(crc, p, chunk);

- Eric

[PATCH v3] lib/crc: arm64: add NEON accelerated CRC64-NVMe implementation

[Demian Shulhan]

Implement an optimized CRC64 (NVMe) algorithm for ARM64 using NEON
Polynomial Multiply Long (PMULL) instructions. The generic shift-and-XOR
software implementation is slow, which creates a bottleneck in NVMe and
other storage subsystems.

The acceleration is implemented using C intrinsics (<arm_neon.h>) rather
than raw assembly for better readability and maintainability.

Key highlights of this implementation:
- Uses 4KB chunking inside scoped_ksimd() to avoid preemption latency
  spikes on large buffers.
- Pre-calculates and loads fold constants via vld1q_u64() to minimize
  register spilling.
- Benchmarks show the break-even point against the generic implementation
  is around 128 bytes. The PMULL path is enabled only for len >= 128.

Performance results (kunit crc_benchmark on Cortex-A72):
- Generic (len=4096): ~268 MB/s
- PMULL (len=4096): ~1556 MB/s (nearly 6x improvement)

Signed-off-by: Demian Shulhan <demyansh@gmail.com>
---
v2: - Removed KERNEL_MODE_NEON check from Kconfig as it's redundant on arm64.
  - Added missing prototype for crc64_nvme_arm64_c to fix sparse/W=1 warning.
  - Improved readability in Makefile with extra newlines and comments.
  - Removed redundant include guards in crc64.h.
  - Switched to do-while loops for better optimization in hot paths.
  - Added comments explaining the magic constants (fold/Barrett).
---
v3: - Removed big-endian fallback from the commit message.
  - Rewrote the comment explaining the final Barrett reduction step.
  - Adjusted the formatting of the scoped_ksimd() call.
---
 lib/crc/Kconfig                  |  1 +
 lib/crc/Makefile                 |  8 +++-
 lib/crc/arm64/crc64-neon-inner.c | 78 ++++++++++++++++++++++++++++++++
 lib/crc/arm64/crc64.h            | 30 ++++++++++++
 4 files changed, 116 insertions(+), 1 deletion(-)
 create mode 100644 lib/crc/arm64/crc64-neon-inner.c
 create mode 100644 lib/crc/arm64/crc64.h

diff --git a/lib/crc/Kconfig b/lib/crc/Kconfig
index 70e7a6016de3..16cb42d5e306 100644
--- a/lib/crc/Kconfig
+++ b/lib/crc/Kconfig
@@ -82,6 +82,7 @@ config CRC64
 config CRC64_ARCH
 	bool
 	depends on CRC64 && CRC_OPTIMIZATIONS
+	default y if ARM64
 	default y if RISCV && RISCV_ISA_ZBC && 64BIT
 	default y if X86_64
 
diff --git a/lib/crc/Makefile b/lib/crc/Makefile
index 7543ad295ab6..c9c35419b39c 100644
--- a/lib/crc/Makefile
+++ b/lib/crc/Makefile
@@ -38,9 +38,15 @@ obj-$(CONFIG_CRC64) += crc64.o
 crc64-y := crc64-main.o
 ifeq ($(CONFIG_CRC64_ARCH),y)
 CFLAGS_crc64-main.o += -I$(src)/$(SRCARCH)
+
+CFLAGS_REMOVE_arm64/crc64-neon-inner.o += -mgeneral-regs-only
+CFLAGS_arm64/crc64-neon-inner.o += -ffreestanding -march=armv8-a+crypto
+CFLAGS_arm64/crc64-neon-inner.o += -isystem $(shell $(CC) -print-file-name=include)
+crc64-$(CONFIG_ARM64) += arm64/crc64-neon-inner.o
+
 crc64-$(CONFIG_RISCV) += riscv/crc64_lsb.o riscv/crc64_msb.o
 crc64-$(CONFIG_X86) += x86/crc64-pclmul.o
-endif
+endif # CONFIG_CRC64_ARCH
 
 obj-y += tests/
 
diff --git a/lib/crc/arm64/crc64-neon-inner.c b/lib/crc/arm64/crc64-neon-inner.c
new file mode 100644
index 000000000000..881cdafadb37
--- /dev/null
+++ b/lib/crc/arm64/crc64-neon-inner.c
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Accelerated CRC64 (NVMe) using ARM NEON C intrinsics
+ */
+
+#include <linux/types.h>
+#include <asm/neon-intrinsics.h>
+
+u64 crc64_nvme_arm64_c(u64 crc, const u8 *p, size_t len);
+
+#define GET_P64_0(v) ((poly64_t)vgetq_lane_u64(vreinterpretq_u64_p64(v), 0))
+#define GET_P64_1(v) ((poly64_t)vgetq_lane_u64(vreinterpretq_u64_p64(v), 1))
+
+/* x^191 mod G, x^127 mod G */
+static const u64 fold_consts_val[2] = { 0xeadc41fd2ba3d420ULL,
+					0x21e9761e252621acULL };
+/* floor(x^127 / G), (G - x^64) / x */
+static const u64 bconsts_val[2] = { 0x27ecfa329aef9f77ULL,
+				    0x34d926535897936aULL };
+
+u64 crc64_nvme_arm64_c(u64 crc, const u8 *p, size_t len)
+{
+	uint64x2_t v0_u64 = { crc, 0 };
+	poly64x2_t v0 = vreinterpretq_p64_u64(v0_u64);
+	poly64x2_t fold_consts =
+		vreinterpretq_p64_u64(vld1q_u64(fold_consts_val));
+	poly64x2_t v1 = vreinterpretq_p64_u8(vld1q_u8(p));
+
+	v0 = vreinterpretq_p64_u8(veorq_u8(vreinterpretq_u8_p64(v0),
+					   vreinterpretq_u8_p64(v1)));
+	p += 16;
+	len -= 16;
+
+	do {
+		v1 = vreinterpretq_p64_u8(vld1q_u8(p));
+
+		poly128_t v2 = vmull_high_p64(fold_consts, v0);
+		poly128_t v0_128 =
+			vmull_p64(GET_P64_0(fold_consts), GET_P64_0(v0));
+
+		uint8x16_t x0 = veorq_u8(vreinterpretq_u8_p128(v0_128),
+					 vreinterpretq_u8_p128(v2));
+
+		x0 = veorq_u8(x0, vreinterpretq_u8_p64(v1));
+		v0 = vreinterpretq_p64_u8(x0);
+
+		p += 16;
+		len -= 16;
+	} while (len >= 16);
+
+	/* Multiply the 128-bit value by x^64 and reduce it back to 128 bits. */
+	poly64x2_t v7 = vreinterpretq_p64_u64((uint64x2_t){ 0, 0 });
+	poly128_t v1_128 = vmull_p64(GET_P64_1(fold_consts), GET_P64_0(v0));
+
+	uint8x16_t ext_v0 =
+		vextq_u8(vreinterpretq_u8_p64(v0), vreinterpretq_u8_p64(v7), 8);
+	uint8x16_t x0 = veorq_u8(ext_v0, vreinterpretq_u8_p128(v1_128));
+
+	v0 = vreinterpretq_p64_u8(x0);
+
+	/* Final Barrett reduction */
+	poly64x2_t bconsts = vreinterpretq_p64_u64(vld1q_u64(bconsts_val));
+
+	v1_128 = vmull_p64(GET_P64_0(bconsts), GET_P64_0(v0));
+
+	poly64x2_t v1_64 = vreinterpretq_p64_u8(vreinterpretq_u8_p128(v1_128));
+	poly128_t v3_128 = vmull_p64(GET_P64_1(bconsts), GET_P64_0(v1_64));
+
+	x0 = veorq_u8(vreinterpretq_u8_p64(v0), vreinterpretq_u8_p128(v3_128));
+
+	uint8x16_t ext_v2 = vextq_u8(vreinterpretq_u8_p64(v7),
+				     vreinterpretq_u8_p128(v1_128), 8);
+
+	x0 = veorq_u8(x0, ext_v2);
+
+	v0 = vreinterpretq_p64_u8(x0);
+	return vgetq_lane_u64(vreinterpretq_u64_p64(v0), 1);
+}
diff --git a/lib/crc/arm64/crc64.h b/lib/crc/arm64/crc64.h
new file mode 100644
index 000000000000..cc65abeee24c
--- /dev/null
+++ b/lib/crc/arm64/crc64.h
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * CRC64 using ARM64 PMULL instructions
+ */
+
+#include <linux/cpufeature.h>
+#include <asm/simd.h>
+#include <linux/minmax.h>
+#include <linux/sizes.h>
+
+u64 crc64_nvme_arm64_c(u64 crc, const u8 *p, size_t len);
+
+#define crc64_be_arch crc64_be_generic
+
+static inline u64 crc64_nvme_arch(u64 crc, const u8 *p, size_t len)
+{
+	if (len >= 128 && cpu_have_named_feature(PMULL) &&
+	    likely(may_use_simd())) {
+		do {
+			size_t chunk = min_t(size_t, len & ~15, SZ_4K);
+
+			scoped_ksimd()
+				crc = crc64_nvme_arm64_c(crc, p, chunk);
+
+			p += chunk;
+			len -= chunk;
+		} while (len >= 128);
+	}
+	return crc64_nvme_generic(crc, p, len);
+}
-- 
2.43.0