Re: [PATCH v4 01/11] timekeeping: add raw clock fallback for random_get_entropy()

[Rob Herring]

On Thu, Apr 14, 2022 at 12:38:49AM +0200, Jason A. Donenfeld wrote:
> Hi Rob,
> 
> On Wed, Apr 13, 2022 at 4:32 PM Rob Herring <robh@xxxxxxxxxx> wrote:
> > 'does not have a usable get_cycles(), ...' as clearly some arches have
> > get_cycles() and yet still need a fallback.
> >
> > Why not handle the 'if get_cycles() returns 0 do the fallback' within
> > a weak random_get_entropy() function? Then more arches don't need any
> > random_get_entropy() implementation.
> 
> No, this doesn't really work. Actually, most archs don't need a
> random_get_entropy() function, because it exists in asm-generic doing
> the thing we want. So that's taken care of. But weak functions as you
> suggested would be quite suboptimal, because on, e.g. x86, what we
> have now gets inlined into a single rdtsc instruction. Also, the
> relation between get_cycles() and random_get_entropy() doesn't always
> hold; some archs may not have a working get_cycles() function but do
> have a path for a random_get_entropy(). Etc, etc. So I'm pretty sure
> that this commit is really the most simple and optimal thing to do. I
> really don't want to go the weak functions route.

Is random_get_entropy() a hot path?


It doesn't have to be a weak function, but look at it this way. We have 
the following possibilities for what random_get_entropy() does:

- get_cycles()
- get_cycles() but returns 0 sometimes
- returns 0
- something else

You're handling the 3rd case.

For the 2nd case, that's riscv, arm, nios2, and x86. That's not a lot, 
but is 2 or 3 of the most widely used architectures. Is it really too 
much to ask to support the 2nd case in the generic code/header?

Rob