Re: [PATCH v3 7/7] pc-bios/s390-ccw: Don't use __bss_start with the "larl" instruction

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu, 29 Jun 2023 13:12:26 +0200
Thomas Huth <thuth@redhat.com> wrote:

> On 29/06/2023 12.58, Claudio Imbrenda wrote:
> > On Thu, 29 Jun 2023 12:48:21 +0200
> > Thomas Huth <thuth@redhat.com> wrote:
> >   
> >> start.S currently cannot be compiled with Clang 16 and binutils 2.40:
> >>
> >>   ld: start.o(.text+0x8): misaligned symbol `__bss_start' (0xc1e5) for
> >>       relocation R_390_PC32DBL
> >>
> >> According to the built-in linker script of ld, the symbol __bss_start
> >> can actually point *before* the .bss section and does not need to have
> >> any alignment, so in certain situations (like when using the internal
> >> assembler of Clang), the __bss_start symbol can indeed be unaligned
> >> and thus it is not suitable for being used with the "larl" instruction
> >> that needs an address that is at least aligned to halfwords.
> >> The problem went unnoticed so far since binutils <= 2.39 did not
> >> check the alignment, but starting with binutils 2.40, such unaligned
> >> addresses are now refused.
> >>
> >> Fix it by loading the address indirectly instead.  
> > 
> > what are the advantages of this solution compared to your previous one
> > (i.e. align .bss) ?  
> 
> __bss_start is supposed to point to an address that is before all bss-like 
> segments. There are also segments like .sbss and .bss.plt on other 
> architectures, see https://bugzilla.redhat.com/show_bug.cgi?id=2216662#c11 .
> Seems like we don't have them on s390x yet, so currently my previous patch 
> is fine, too. But in case there will ever be an extension to the s390x ABI 
> that introduces such additional segments, we have to switch back to 
> __bss_start again. So it sounds slightly more future-proof to me to keep 
> __bss_start here, even if we need a slightly more complex startup code here now.

fair enough

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> 
>   Thomas
> 
>