Re: git: cade8f6c118f - main - ELF auxargs: reserve a slot for CheriABI use

On Tue, Jul 23, 2024 at 09:44:02PM +0000, Brooks Davis wrote:
> The branch main has been updated by brooks:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=cade8f6c118f304eb7c91a1d423b4a97ee466284
> 
> commit cade8f6c118f304eb7c91a1d423b4a97ee466284
> Author:     Brooks Davis <brooks@FreeBSD.org>
> AuthorDate: 2024-07-23 21:31:44 +0000
> Commit:     Brooks Davis <brooks@FreeBSD.org>
> CommitDate: 2024-07-23 21:31:44 +0000
> 
>     ELF auxargs: reserve a slot for CheriABI use
>     
>     Reserve slot 37 to avoid flag days downstream.
>     
>     We currently use this slot in CheriBSD to maintain counters in userspace
>     while providing access to them vis sysctl.  CHERI support isn't landing
>     in FreeBSD quite yet (probably for 16 unless hardware access accelerates
>     radically), but it seems highly likely at this point so reserving slots
>     will limit future diffs in both directions.

In anticipation of merging CHERI support over the next few years, we're
going to start reserving values like this where we're confident in the
functionality they represent and that the values will hold (either because
they are under our control or because some upstream standards body like
RISC-V International has allocated them.)  I've started here because
andrew@ noticed that Linux now has AT_HWCAP{3,4} defines and it would be
disruptive for us downstream if there was a collision.

-- Brooks