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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 6 Oct 2019 11:26:54 -0700
Message-ID: <CAHk-=whT=S76NWHVfvvV7Vcx2G+e-QAB=89jykA20a4TRYAT+g@mail.gmail.com>
Subject: Re: x86/random: Speculation to the rescue
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        "Ahmed S. Darwish" <darwish.07@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Nicholas Mc Guire <hofrat@opentech.at>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>
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On Sun, Oct 6, 2019 at 11:21 AM Pavel Machek <pavel@ucw.cz> wrote:
>
>
> Even without cycle counter... if we _know_ we are trying to generate
> entropy and have MMC available, we don't care about power and
> performance.
>
> So we can just...
>
>    issue read request on MMC
>    while (!interrupt_done)
>          i++
>
> ...and then use i++ as poor man's version of cycle counter.
>
> [We would not want to do that in normal operation, for obvious
> reasons, just when userland is blocked and waiting for entropy.]
>
> Hmm?

I hate it, but it might be worth it for the existing timer thing
alternative when we don't have a cycle counter.

Then we'd have _something_ for those bad embedded devices.

I still absolutely hate the idea of doing disk IO for this.

               Linus