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Subject: Re: RISC-V nommu support v2
To:     Christoph Hellwig <hch@lst.de>
Cc:     Palmer Dabbelt <palmer@sifive.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        linux-riscv@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20190624054311.30256-1-hch@lst.de>
 <28e3d823-7b78-fa2b-5ca7-79f0c62f9ecb@arm.com> <20190624115428.GA9538@lst.de>
From:   Vladimir Murzin <vladimir.murzin@arm.com>
Message-ID: <d4fd824d-03ff-e8ab-b19f-9e5ef5c22449@arm.com>
Date:   Mon, 24 Jun 2019 14:08:50 +0100
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On 6/24/19 12:54 PM, Christoph Hellwig wrote:
> On Mon, Jun 24, 2019 at 12:47:07PM +0100, Vladimir Murzin wrote:
>> Since you are using binfmt_flat which is kind of 32-bit only I was expecting to see
>> CONFIG_COMPAT (or something similar to that, like ILP32) enabled, yet I could not
>> find it.
> 
> There is no such thing in RISC-V.  I don't know of any 64-bit RISC-V
> cpu that can actually run 32-bit RISC-V code, although in theory that
> is possible.  There also is nothing like the x86 x32 or mips n32 mode
> available either for now.
> 
> But it turns out that with a few fixes to binfmt_flat it can run 64-bit
> binaries just fine.  I sent that series out a while ago, and IIRC you
> actually commented on it.
> 

True, yet my observation was that elf2flt utility assumes that address
space cannot exceed 32-bit (for header and absolute relocations). So,
from my limited point of view straightforward way to guarantee that would
be to build incoming elf in 32-bit mode (it is why I mentioned COMPAT/ILP32).

Also one of your patches expressed somewhat related idea

"binfmt_flat isn't the right binary format for huge executables to
start with"

Since you said there is no support for compat/ilp32, probably I'm missing some
toolchain magic?

Cheers
Vladimir