linux-kernel - RE: [patch 3/3] x86/fpu: Make FPU protection more robust

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Date:   Thu, 5 May 2022 11:34:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Jason A. Donenfeld'" <Jason@...c4.com>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Filipe Manana <fdmanana@...e.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>
Subject: RE: [patch 3/3] x86/fpu: Make FPU protection more robust

...
> +	cycles_t end, start = get_cycles();
>  	blake2s_update(&input_pool.hash, in, nbytes);
> +	end = get_cycles();

If get_cycles() is rdtsc then that gives meaningless numbers.
The cpu clock frequency will change on you.

You can use one of the performance counters to get an actual
cycle count - although that is only stable for 'hot cache'
as any memory accesses are clock speed dependant.

OTOH the entropy mixing is very likely to be 'cold cache'
and all the unrolling in blakes7 will completely kill
performance.

	David

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