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

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Date:   Sun, 11 Mar 2018 14:00:13 +0200
From:   Nikolay Borisov <n.borisov.lkml@...il.com>
To:     Andiry Xu <jix024@....ucsd.edu>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org
Cc:     dan.j.williams@...el.com, andy.rudoff@...el.com,
        coughlan@...hat.com, swanson@...ucsd.edu, david@...morbit.com,
        jack@...e.com, swhiteho@...hat.com, miklos@...redi.hu,
        andiry.xu@...il.com, Andiry Xu <jix024@...ucsd.edu>,
        herbert@...dor.apana.org.au
Subject: Re: [RFC v2 05/83] Add NOVA filesystem definitions and useful helper
 routines.

[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>