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
