linux-kernel - Re: [PATCH 2/2] arm64/crc32: Implement 4-way interleave using PMULL

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Message-ID: <20241016162411.GA3228925@google.com>
Date: Wed, 16 Oct 2024 16:24:11 +0000
From: Eric Biggers <ebiggers@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-crypto@...r.kernel.org, herbert@...dor.apana.org.au,
	will@...nel.org, catalin.marinas@....com,
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH 2/2] arm64/crc32: Implement 4-way interleave using PMULL

On Wed, Oct 16, 2024 at 09:12:41AM +0200, Ard Biesheuvel wrote:
> > I'd recommend calling the file crc32-4way.S and the functions
> > crc32*_arm64_4way(), rather than crc32-pmull.S and crc32*_pmull().  This would
> > avoid confusion with a CRC implementation that is actually based entirely on
> > pmull (which is possible).
> 
> I'm well aware :-)
> 
> commit 8fefde90e90c9f5c2770e46ceb127813d3f20c34
> Author: Ard Biesheuvel <ardb@...nel.org>
> Date:   Mon Dec 5 18:42:27 2016 +0000
> 
>     crypto: arm64/crc32 - accelerated support based on x86 SSE implementation
> 
> commit 598b7d41e544322c8c4f3737ee8ddf905a44175e
> Author: Ard Biesheuvel <ardb@...nel.org>
> Date:   Mon Aug 27 13:02:45 2018 +0200
> 
>     crypto: arm64/crc32 - remove PMULL based CRC32 driver
> 
> I removed it because it wasn't actually faster, although that might be
> different on modern cores.

The PMULL-based code removed by commit 598b7d41e544 was only 4-wide.  On
Apple M1, a 12-wide PMULL-based CRC32 is actually faster than 4-way CRC32,
especially if the eor3 instruction from the sha3 extension is utilized.

This was not the case on non-Apple CPUs I tested (in 2022), though.  12-wide is
very wide and is a bit inconvenient, and IMO it's not worth doing in the kernel
at this point.  It would be interesting to test the very latest CPUs, though.

- Eric