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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 5 November 2017 at 05:52, Leif Lindholm <leif.lindholm@linaro.org> wrote:
> On Fri, Nov 03, 2017 at 11:33:52AM +0000, Ard Biesheuvel wrote:
>> DEBUG builds of PEI code will print a diagnostic message regarding
>> the utilization of temporary RAM before switching to permanent RAM.
>> For example,
>>
>>   Total temporary memory:    16352 bytes.
>>     temporary memory stack ever used:       4820 bytes.
>>     temporary memory heap used for HobList: 4720 bytes.
>>
>> Tracking stack utilization like this requires the stack to be seeded
>> with a known magic value, and this needs to occur before entering C
>> code, given that it uses the stack. Currently, only Nt32Pkg appears
>> to implement this feature, but it is useful nonetheless, so let's
>> wire it up for PrePeiCore as well.
>>
>> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=748
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
> 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.