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Date:   Thu, 14 Apr 2022 11:27:22 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
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Subject: Re: [PATCH v4 04/11] mips: use fallback for random_get_entropy()
 instead of zero
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Hi Maciej,

On Thu, Apr 14, 2022 at 02:16:18AM +0100, Maciej W. Rozycki wrote:
>  Yes, for the relevant CPUs the range is 63-8 << 8 for R3k machines and 
> 47-0 (the lower bound can be higher if wired entries are used, which I 
> think we occasionally do) for R4k machines with a buggy CP0 counter.  So 
> there are either 56 or up to 48 distinct CP0 Random register values.

Ahh interesting, so it varies a bit, but it remains rather small.

>  It depends on the exact system.  Some have a 32-bit high-resolution 
> counter in the chipset (arch/mips/kernel/csrc-ioasic.c) giving like 25MHz 
> resolution, some have nothing but jiffies.

Alright, so there _are_ machines with no c0 cycles but with a good
clock. Yet, 25MHz is still less than the cpu cycle, so this c0 random
ORing trick remains useful perhaps.

>  It seems like a reasonable idea to me, but the details would have to be 
> sorted out, because where a chipset high-resolution counter is available 
> we want to factor it in, and otherwise we need to extract the right bits 
> from the CP0 Random register, either 13:8 for the R3k or 5:0 for the R4k.

One thing we could do here that would seemingly cover all the cases
without losing _that_ much would be:

    return (random_get_entropy_fallback() << 13) | ((1<<13) - read_c0_random());

Or in case the 13 turns out to be wrong on some hardware, we could
mitigate the effect with:

    return (random_get_entropy_fallback() << 13) ^ ((1<<13) - read_c0_random());

As mentioned in the 1/xx patch of this series,
random_get_entropy_fallback() should call the highest resolution thing.
We then shave off the least-changing bits and stuff in the
faster-changing bits from read_c0_random(). Then, in order to keep it
counting up instead of down, we do the subtraction there.

What do you think of this plan?

Jason