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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Thu, 5 May 2022 13:35:49 +0200
Message-ID: <CAHmME9q+1dAg=H2RLDHd=CSCwO4PpL+YYMeDXO6uQ_wD+GNPhg@mail.gmail.com>
Subject: Re: [patch 3/3] x86/fpu: Make FPU protection more robust
To:     David Laight <David.Laight@aculab.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        LKML <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        Filipe Manana <fdmanana@suse.com>,
        "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>
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On Thu, May 5, 2022 at 1:34 PM David Laight <David.Laight@aculab.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