[PATCH 2/6] arm64: Allow mismatched 32-bit EL0 support

[Catalin Marinas]

On Thu, Nov 05, 2020 at 09:38:46PM +0000, Will Deacon wrote:
> Ok. Then we're in agreement about not preventing late-onlining. The problem
> then is that the existing 32-bit EL0 capability is a SYSTEM cap so even with
> your diff, we still have an issue if you boot on the CPUs that support
> 32-bit and then try to online a 64-bit-only core (it will fail).

Ah, I focussed too much on the 32-bit capable CPUs coming up late. In my
original hack, I made the capability weak based on the config option.
Here we want to make it weak based on cmdline but that structure is
const (we could remove the const though).

> So I think we do need my changes to the existing cap, but perhaps we
> could return false from system_supports_32bit_el0() until we've actually
> seen a 32-bit capable core. That way you would keep the existing behaviour
> on TX2, and we wouldn't get any unusual late-onlining failures.

If we see the first 32-bit capable core late, we may report it's
available but no proper hwcaps.

We could do a combination of a new weak feature together with your
always-on 32-bit feature when forced by the cmdline. So the system would
support 32-bit if both the system feature (with the detection override)
and the asym one are set.

However, I think it may be simpler if we made the current feature weak
(so no new one) together with a bool somewhere that tells us if we found
a CPU that doesn't have 32-bit (asym mode). system_supports_32bit_el0()
would check if the cap is set together with (!asym_found ||
asym_allowed).

-- 
Catalin