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Date:   Mon, 18 Jul 2022 13:52:38 +1000
From:   Michael Ellerman <>
To:     Segher Boessenkool <>,
        Linus Torvalds <>
Cc:     Sudip Mukherjee <>,
        Kees Cook <>,
        linux-kernel <>,
        Paul Mackerras <>,,
        linuxppc-dev <>
Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check

Segher Boessenkool <> writes:
> On Sun, Jul 17, 2022 at 07:44:22AM -0700, Linus Torvalds wrote:
>> On Sun, Jul 17, 2022 at 2:13 AM Sudip Mukherjee
>> <> wrote:
>> > I was trying to check it. With gcc-11 the assembly code generated is
>> > not using memset, but using __memset.
>> > But with gcc-12, I can see the assembly code is using memset. One
>> > example from the assembly:
>> You could try making the 'args' array in 'struct prom_args' be marked
>> 'volatile'.
>> Ie something like this:
>>   --- a/arch/powerpc/kernel/prom_init.c
>>   +++ b/arch/powerpc/kernel/prom_init.c
>>   @@ -115,6 +115,6 @@ struct prom_args {
>>            __be32 service;
>>            __be32 nargs;
>>            __be32 nret;
>>   -          __be32 args[10];
>>   +        volatile __be32 args[10];
>>    };
>> because I think it's just the compilers turning the small loop over
>> those fields into a "memset()".
> Yes.  See <>
> near the end:
>   Most of the compiler support routines used by GCC are present in
>   libgcc, but there are a few exceptions. GCC requires the freestanding
>   environment provide memcpy, memmove, memset and memcmp. Finally, if
>   __builtin_trap is used, and the target does not implement the trap
>   pattern, then GCC emits a call to abort.
> Can't we simply have a small simple implementation of these functions in
> arch/powerpc/boot/?  This stuff is not performance-critical, and this is
> not the first time we hit these problems.

prom_init.c isn't in arch/powerpc/boot :)

It's linked into the kernel proper, but we want it to behave like a
pre-boot environment (because not all boot paths run it) which is why we
restrict what symbols it can call.

We could have a prom_memset() etc. but we'd need to do some tricks to
rewrite references to memset() to prom_memset() before linking.


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