Re: [kvm-unit-tests PATCH] s390x: Ignore gcc 12 warnings for low addresses

Thomas Huth
2022-05-17T16:10:05Z

On 17/05/2022 14.02, Claudio Imbrenda wrote:
> On Mon, 16 May 2022 16:43:32 +0200
> Janis Schoetterl-Glausch <scgl@linux.ibm.com> wrote:
>
>> gcc 12 warns if a memory operand to inline asm points to memory in the
>> first 4k bytes. However, in our case, these operands are fine, either
>> because we actually want to use that memory, or expect and handle the
>> resulting exception.
>> Therefore, silence the warning.
>
> I really dislike this

I agree the pragmas are ugly. But maybe we should mimic what the kernel
is doing here?

$ git show 8b202ee218395
commit 8b202ee218395319aec1ef44f72043e1fbaccdd6
Author: Sven Schnelle <svens@linux.ibm.com>
Date: Mon Apr 25 14:17:42 2022 +0200

s390: disable -Warray-bounds

gcc-12 shows a lot of array bound warnings on s390. This is caused
by the S390_lowcore macro which uses a hardcoded address of 0.

Wrapping that with absolute_pointer() works, but gcc no longer knows
that a 12 bit displacement is sufficient to access lowcore. So it
emits instructions like 'lghi %r1,0; l %rx,xxx(%r1)' instead of a
single load/store instruction. As s390 stores variables often
read/written in lowcore, this is considered problematic. Therefore
disable -Warray-bounds on s390 for gcc-12 for the time being, until
there is a better solution.

... so we should maybe disable it in the Makefile, too, until the
kernel folks found a nicer solution?

Thomas