linux-kernel - RE: [PATCH] arm64: crypto: Add an option to assume NEON XOR is the fastest

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Message-ID: <ad9108e155ba4245a2005e9212a7d2b5@AcuMS.aculab.com>
Date:   Tue, 22 Sep 2020 08:25:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Douglas Anderson' <dianders@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
CC:     Jackie Liu <liuyun01@...inos.cn>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] arm64: crypto: Add an option to assume NEON XOR is the
 fastest

From: Douglas Anderson
> Sent: 22 September 2020 01:26
> 
> On every boot time we see messages like this:
> 
> [    0.025360] calling  calibrate_xor_blocks+0x0/0x134 @ 1
> [    0.025363] xor: measuring software checksum speed
> [    0.035351]    8regs     :  3952.000 MB/sec
> [    0.045384]    32regs    :  4860.000 MB/sec
> [    0.055418]    arm64_neon:  5900.000 MB/sec
> [    0.055423] xor: using function: arm64_neon (5900.000 MB/sec)
> [    0.055433] initcall calibrate_xor_blocks+0x0/0x134 returned 0 after 29296 usecs
> 
> As you can see, we spend 30 ms on every boot re-confirming that, yet
> again, the arm64_neon implementation is the fastest way to do XOR.
> ...and the above is on a system with HZ=1000.  Due to the way the
> testing happens, if we have HZ defined to something slower it'll take
> much longer.  HZ=100 means we spend 300 ms on every boot re-confirming
> a fact that will be the same for every bootup.

Can't the code use a TSC (or similar high-res counter) to
see how long it takes to process a short 'hot cache' block?
That wouldn't take long at all.

	David

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