Re: [PATCH v2 4/5] arm64: Add support for SMCCC TRNG entropy source

[Mark Rutland]

On Thu, Nov 05, 2020 at 03:34:01PM +0100, Ard Biesheuvel wrote:
> On Thu, 5 Nov 2020 at 15:30, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > On Thu, Nov 05, 2020 at 03:04:57PM +0100, Ard Biesheuvel wrote:
> > > On Thu, 5 Nov 2020 at 15:03, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >
> > > > That said, I'm not sure it's great to plumb this under the
> > > > arch_get_random*() interfaces, e.g. given this measn that
> > > > add_interrupt_randomness() will end up trapping to the host all the time
> > > > when it calls arch_get_random_seed_long().
> > >
> > > As it turns out, add_interrupt_randomness() isn't actually used on ARM.
> >
> > It's certainly called on arm64, per a warning I just hacked in:

[...]

> > ... and I couldn't immediately spot why 32-bit arm  would be different.
> 
> Hmm, I actually meant both arm64 and ARM.
> 
> Marc looked into this at my request a while ago, and I had a look
> myself as well at the time, and IIRC, we both concluded that we don't
> hit that code path. Darn.
> 
> In any case, the way add_interrupt_randomness() calls
> arch_get_random_seed_long() is absolutely insane, so we should try to
> fix that in any case.

I have no strong opinion there, and I'm happy with that getting cleaned
up.

Regardless, I do think it's reasonable for the common code to expect
that arch_get_random_*() to be roughly as expensive as "most other
instructions" (since even RNDR* is expensive the CPU might be able to do
useful speculative work in the mean time), whereas a trap to the host is
always liable to be expensive as no useful work can be done while the
host is handling it, so I think it makes sense to distinguish the two.

Thanks,
Mark.