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    SubjectRe: [PATCH] /dev/random: Insufficient of entropy on many architectures
    Date
    Am Donnerstag, 12. September 2013, 14:15:35 schrieb Geert Uytterhoeven:

    Hi Geert,

    >On Thu, Sep 12, 2013 at 2:08 PM, Stephan Mueller <smueller@chronox.de>
    wrote:
    >>>BTW, I prefer a different name than "random_get_fast_cycles()", as
    >>>it's better to have something that returns different and
    >>>unpredictable numbers than an actual monotonic cycle counter.
    >>>
    >> A monotonic counter is fully ok. Note, for /dev/random, the
    >> occurrence
    >> of events delivers entropy. Thus, we have to be able to precisely
    >> measure that occurrence. The timer itself does not need to deliver
    >> any
    >> entropy as long as it is fast.
    >
    >Well, in my specific case (m68k/Amiga) I can use:
    > - a 24-bit counter running at only ca. 15 or 31 kHz (actual
    >frequency may vary),
    > - a 16-bit counter running at ca. 700 kHz.
    >
    >That is, if they have to be monotonic cycle counters.
    >
    >If not, I can mix the two (e.g. "a << 8 | (b & 0xff)") to get a 32-bit
    >value. That result would be fine for /dev/random, I guess, but it's
    >not really "get_cycles()".

    Note, get_cycles should return an u64.

    Not sure what a and b here is, but if a is the 24 bit value and b the
    faster 16 bit value, wouldn't there be a gap?

    I.e. wouldn't it be better to use the full 16 bit counter as low value
    and OR the 24 bit on bits 48 to 17?

    Yet, there is a break in that counter: the 16 low bits rotate several
    times (around 10 times) before bit 17 is changed once.


    >
    >Gr{oetje,eeting}s,
    >
    > Geert
    >
    >--
    >Geert Uytterhoeven -- There's lots of Linux beyond ia32 --
    >geert@linux-m68k.org
    >
    >In personal conversations with technical people, I call myself a
    >hacker. But when I'm talking to journalists I just say "programmer" or
    >something like that. -- Linus Torvalds


    Ciao
    Stephan


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