linux-kernel - Re: [PATCH v8 1/3] riscv: Introduce CONFIG

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Date:   Fri, 24 Mar 2023 11:34:26 +0100
From:   Alexandre Ghiti <alex@...ti.fr>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Björn Töpel <bjorn@...nel.org>
Cc:     Alexandre Ghiti <alexghiti@...osinc.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, nathan@...nel.org,
        linux-kbuild@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v8 1/3] riscv: Introduce CONFIG_RELOCATABLE

Hi Nick,

On 3/22/23 19:25, Nick Desaulniers wrote:
> On Fri, Feb 24, 2023 at 7:58 AM Björn Töpel <bjorn@...nel.org> wrote:
>> Alexandre Ghiti <alex@...ti.fr> writes:
>>
>>> +cc linux-kbuild, llvm, Nathan, Nick
>>>
>>> On 2/15/23 15:36, Alexandre Ghiti wrote:
>>>> From: Alexandre Ghiti <alex@...ti.fr>
>>>>
>>> I tried a lot of things, but I struggle to understand, does anyone have
>>> any idea? FYI, the same problem happens with LLVM.
> Off the top of my head, no idea.
>
> (Maybe as a follow up to this series, I wonder if pursuing
> ARCH_HAS_RELR for ARCH=riscv is worthwhile?)


IIUC, the goal for using RELR is to reduce the size of a kernel image: 
right now, this is not my priority, but I'll add that to my todo list 
because that may be useful to distros.


>
>> Don't ask me *why*, but adding --emit-relocs to your linker flags solves
>> "the NULL .rela.dyn" both for GCC and LLVM.
>>
>> The downside is that you end up with a bunch of .rela cruft in your
>> vmlinux.
> There was a patch just this week to use $(OBJCOPY) to strip these from
> vmlinux (for x86). Looks like x86 uses --emit-relocs for KASLR:
> https://lore.kernel.org/lkml/20230320121006.4863-1-petr.pavlu@suse.com/


That's nice, that would be an interesting intermediate step until we 
find the issue here as I believe it is important to have the relocations 
in the init section to save memory.

Thanks for your answer Nick, really appreciated,

Alex