Re: [PATCH] arm: fix u-boot port syscall interface va_arg handling (was Re: [PATCH] [ARM] Enable boot module for arm)

[Andrey Borzenkov <arvidjaar@gmail.com>]

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В Sat, 16 Nov 2013 14:17:51 +0100
Vladimir 'φ-coder/phcoder' Serbinenko <phcoder@gmail.com> пишет:

> On 16.11.2013 14:00, Leif Lindholm wrote:
> > On Sat, Nov 16, 2013 at 01:49:51PM +0100, Vladimir 'φ-coder/phcoder' Serbinenko wrote:
> >> On 16.11.2013 13:39, Leif Lindholm wrote:
> >>> -	str     r9, transition_space + 8
> >> You need to save r9. Otherwise GRUB will crash if compiled with clang.
> > 
> > Hmm, but this is a bug in clang in that case.
> > 
> > From an ABI perspective, grub_uboot_syscall is a veneer:
> > the only visible side effect it is permitted to have is to corrupt r12.
> > 
> According to wikipedia:
> "Subroutines must preserve the contents of r4 to r11 and the stack pointer."

According to ARM ABI documentation

"A subroutine must preserve the contents of the registers r4 - r8, r10,
r11 and SP (and r9 in PCS variants that designate r9 as v6)."

So it sounds like whether r9 is preserved depends on platform.

> So changing r9 sounds to me like this is actually U-Boot bug and
> preserving it sounds like right way to handle it.
> What's the harm in preserving r9? Is it used for something in calling?
> > We need to additionally switch r8 between grub and u-boot copy because
> > u-boot uses it for its global data pointer, and lr because we need to
> > corrupt it to make u-boot return to this veneer to switch the r8 back
> > again.
> > 
> > This veneer should not touch anything else - so if we need to
> > temporarily work around a toolchain ABI bug, can we do this via an
> > ifdef and a config option?
> > 
> If the bug is on clang side (I doubt it right now), there is no way to
> handle it without side effects and we can invent a test for it, we can
> reject buggy compilers (clang is secondary compiler)
> > /
> >     Leif
> > 
> > _______________________________________________
> > Grub-devel mailing list
> > Grub-devel@gnu.org
> > https://lists.gnu.org/mailman/listinfo/grub-devel
> > 
> 
> 


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