Re: [PATCH v2] ArmPlatformPkg/PrePeiCore: seed temporary stack before entering PEI core

[Leif Lindholm <leif.lindholm@linaro.org>]

On Sun, Nov 05, 2017 at 04:29:15PM +0000, Ard Biesheuvel wrote:
> >> OK, this may sound completely unreasonable, but seeing those
> >> implementations overwrite callee-saved registers without saving them
> >> makes my brain unhappy. (Yes, I know.)
> >>
> >> Could they either:
> >> - Have a comment prepended establishing the implicit ABI of which
> >>   registers the caller cannot rely on reusing after return.
> >>   Preferably somewhat echoed at the call site.
> >> - Be rewritten to use only scratch registers?
> >>
> >
> > I think it is implied that the startup code does not adhere to the
> > AAPCS. That code already uses r5 and r6 without stacking them, simply
> > because we're in the middle of preparing the stack and other execution
> > context, precisely so the C code we call into can rely on AAPCS
> > guarantees.
> 
> Ehm, hold on, what do you mean by 'call site'? This code just runs and
> jumps back to a local label. There are no functions calls here until
> the point where we call into C (with the exception of the lovely
> ArmPlatformPeiBootAction() we added so Juno can find out how much DRAM
> it can use)

Yeah, you're right, I was misreading the block as a subroutine.

Seems the only register that must be preserved across jumps is r5/x5,
and neither of these modifications touch those (or change that fact).

Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

/
    Leif