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SubjectRe: x86/random: Speculation to the rescue
On Sun 2019-10-06 11:06:38, Linus Torvalds wrote:
> On Sun, Oct 6, 2019 at 10:35 AM Pavel Machek <pavel@ucw.cz> wrote:
> >
> > It will not: boot is now halted because systemd wants some
> > entropy. Everything is idle and very little interrupts are
> > happening. We have spinning rust, but it is idle, and thus not
> > generating any interrupts.
>
> Yes, but we have that problem now solved.
>
> Except on embedded platforms that have garbage CPU's without even a
> cycle counter.
>
> But those won't have spinning rust anyway.
>
> Yes, bad SSD's and MMC disks (that they do have) will generate timing
> noise too, but in the absense of a cycle counter, that noise won't be
> much use.

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?

Best regards,
Pavel

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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