linux-kernel - Re: RISC-V nommu support v2

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Date:   Mon, 24 Jun 2019 14:08:50 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Damien Le Moal <damien.lemoal@....com>,
        linux-riscv@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: RISC-V nommu support v2

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