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

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Date:   Thu, 5 May 2022 13:35:49 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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

On Thu, May 5, 2022 at 1:34 PM David Laight <David.Laight@...lab.com> wrote:
>
> ...
> > +     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

Indeed. In the process of wiring up rdpmc now.

Jason