Re: [RFC v2 05/83] Add NOVA filesystem definitions and useful helper routines.

Nikolay Borisov
2018-03-11T12:00:19Z

[Adding Herbert Xu to CC since he is the maintainer of the crypto subsys
maintainer]

On 10.03.2018 20:17, Andiry Xu wrote:
<snip>

> +static inline u32 nova_crc32c(u32 crc, const u8 *data, size_t len)
> +{
> + u8 *ptr = (u8 *) data;
> + u64 acc = crc; /* accumulator, crc32c value in lower 32b */
> + u32 csum;
> +
> + /* x86 instruction crc32 is part of SSE-4.2 */
> + if (static_cpu_has(X86_FEATURE_XMM4_2)) {
> + /* This inline assembly implementation should be equivalent
> + * to the kernel's crc32c_intel_le_hw() function used by
> + * crc32c(), but this performs better on test machines.
> + */
> + while (len > 8) {
> + asm volatile(/* 64b quad words */
> + "crc32q (%1), %0"
> + : "=r" (acc)
> + : "r" (ptr), "0" (acc)
> + );
> + ptr += 8;
> + len -= 8;
> + }
> +
> + while (len > 0) {
> + asm volatile(/* trailing bytes */
> + "crc32b (%1), %0"
> + : "=r" (acc)
> + : "r" (ptr), "0" (acc)
> + );
> + ptr++;
> + len--;
> + }
> +
> + csum = (u32) acc;
> + } else {
> + /* The kernel's crc32c() function should also detect and use the
> + * crc32 instruction of SSE-4.2. But calling in to this function
> + * is about 3x to 5x slower than the inline assembly version on
> + * some test machines.

That is really odd. Did you try to characterize why this is the case? Is
it purely the overhead of dispatching to the correct backend function?
That's a rather big performance hit.

> + */
> + csum = crc32c(crc, data, len);
> + }
> +
> + return csum;
> +}
> +

<snip>