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From:   Alex Ghiti <alex@ghiti.fr>
Subject: Re: [PATCH v5 2/4] riscv: Introduce CONFIG_RELOCATABLE
To:     Jerome Forissier <jerome@forissier.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Anup Patel <Anup.Patel@wdc.com>,
        Atish Patra <Atish.Patra@wdc.com>,
        Zong Li <zong.li@sifive.com>, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org
Cc:     Anup Patel <anup@brainfault.org>
References: <20200607075949.665-1-alex@ghiti.fr>
 <20200607075949.665-3-alex@ghiti.fr>
 <b588dd9e-dff8-3458-0c7d-149e3990bca7@forissier.org>
Message-ID: <43ef1623-2186-aa07-cfa5-d618aa4f09c0@ghiti.fr>
Date:   Thu, 11 Jun 2020 15:43:56 -0400
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Hi Jerome,

Le 6/10/20 à 10:10 AM, Jerome Forissier a écrit :
> On 6/7/20 9:59 AM, Alexandre Ghiti wrote:
> [...]
>
>> +config RELOCATABLE
>> +	bool
>> +	depends on MMU
>> +	help
>> +          This builds a kernel as a Position Independent Executable (PIE),
>> +          which retains all relocation metadata required to relocate the
>> +          kernel binary at runtime to a different virtual address than the
>> +          address it was linked at.
>> +          Since RISCV uses the RELA relocation format, this requires a
>> +          relocation pass at runtime even if the kernel is loaded at the
>> +          same address it was linked at.
> Is this true? I thought that the GNU linker would write the "proper"
> values by default, contrary to the LLVM linker (ld.lld) which would need
> a special flag: --apply-dynamic-relocs (by default the relocated places
> are set to zero). At least, it is my experience with Aarch64 on a
> different project. So, sorry if I'm talking nonsense here -- I have not
> looked at the details.
>
>

It seems that you're right, at least for aarch64 since they specifically 
specify the --no-apply-dynamic-relocs option. I retried to boot without 
relocating at runtime, and it fails on riscv. Can this be arch specific ?

Thanks,

Alex