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

[H. Peter Anvin]

On April 14, 2022 1:41:38 PM PDT, Rob Herring <robh at kernel.org> wrote:
>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 at kernel.org> 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

It goes into interrupts, which means it is latency critical.