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Date:   Thu, 14 Apr 2022 02:16:18 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@orcam.me.uk>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
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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 Jason,

> However, one thing that I've been thinking about is that the c0 random
> register is actually kind of garbage. In my fuzzy decade-old memory of
> MIPS, I believe the c0 random register starts at the maximum number of
> TLB entries (16?), and then counts down cyclically, decrementing once
> per CPU cycle. Is that right?

 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.

> If it is, there are some real pros and cons here to consider:
> - Pro: decrementing each CPU cycle means pretty good granularity
> - Con: wrapping at, like, 16 or something really is very limited, to
> the point of being almost bad
> 
> Meanwhile, on systems without the c0 cycles counter, what is the
> actual resolution of random_get_entropy_fallback()? Is this just
> falling back to jiffies?

 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.

> IF (a) the fallback is jiffies AND (b) c0 wraps at 16, then actually,
> what would be really nice would be something like:
> 
>     return (jiffies << 4) | read_c0_random();
> 
> It seems like that would give us something somewhat more ideal than
> the status quo. Still crap, of course, but undoubtedly better.

 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.

  Maciej