[PATCH 0/3] ARM: v7: get rid of boot time mini stack

[Nicolas Pitre]

On Mon, 8 Feb 2021, Ard Biesheuvel wrote:

> The v7 boot code uses a small chunk of BSS to preserve some register
> contents across a call to v7_invalidate_l1 that occurs with the MMU and
> caches disabled. Memory accesses in such cases are tricky on v7+, given
> that the architecture permits some unintuitive behaviors (it is
> implementation defined whether accesses done with the MMU and caches off
> may hit in the caches, and on SoCs that incorporate off-core system
> caches, this behavior appears to be different even between cache
> levels). Also, cache invalidation is not safe under virtualization if
> the intent is to retain stores issued directly to DRAM, given that the
> hypervisor may upgrade invalidate operations to clean+invalidate,
> resulting in DRAM contents to be overwritte by the dirty cachelines that
> we were trying to evict in the first place.
> 
> So let's address this issue, by removing the need for this stack to
> exist in the first place: v7_invalidate_l1 can be rewritten to use fewer
> registers, which means fewer registers need to be preserved, and we have
> enough spare registers available.

That is excellent.

I wonder why r1-r3 were preserved though.


Nicolas