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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Sun, 26 Dec 2021 15:31:22 -0500
Message-ID: <CAK8P3a08kawHpz6RsLZsvNNn0_WKaFWRU-7M4fJ3e7i7J+GQcA@mail.gmail.com>
Subject: Re: [PATCH 00/13] riscv: compat: Add COMPAT mode support for rv64
To:     Guo Ren <guoren@kernel.org>
Cc:     Jisheng Zhang <jszhang3@mail.ustc.edu.cn>,
        Arnd Bergmann <arnd@arndb.de>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Anup Patel <anup.patel@wdc.com>,
        gregkh <gregkh@linuxfoundation.org>,
        liush <liush@allwinnertech.com>, Wei Fu <wefu@redhat.com>,
        Drew Fustini <drew@beagleboard.org>,
        Wang Junqiang <wangjunqiang@iscas.ac.cn>,
        =?UTF-8?B?V2VpIFd1ICjlkLTkvJ8p?= <lazyparser@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        linux-csky@vger.kernel.org, Guo Ren <guoren@linux.alibaba.com>
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On Sun, Dec 26, 2021 at 7:38 AM Guo Ren <guoren@kernel.org> wrote:
> On Sun, Dec 26, 2021 at 4:36 PM Jisheng Zhang <jszhang3@mail.ustc.edu.cn> wrote:
> > On Wed, 22 Dec 2021 20:59:30 +0800 Guo Ren <guoren@kernel.org> wrote:
> > > On Wed, Dec 22, 2021 at 2:10 AM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > What about adding RV64 ILP32 support instead? This don't need HW side
> > modifications so can benefit all RV64.
>
> ILP32 is another topic in C Language Data Type Models and it couldn't
> replace the standard rv32 ecosystem.
> COMPAT is a common framework in Linux (7 arches have been supported),
> so let rv64 support COMPAT mode is considerable.
>
> Customers would choose ILP32 / RV32-compat by themself and that
> depends on which one has a better ecosystem.

From a kernel perspective, supporting both is not much more work than
supporting either of them. We had the same debate for Arm64, and ended
up never merging the ILP32 patches despite them being well written
and maintainable, to limit the number of supported user space ABIs
as well as the possible attack vectors when there is an exploitable
bug that is specific to an ABI.

arm64 does support big-endian mode, which is a similar niche, but it
can't easily be removed after it's already supported. Supporting normal
compat mode is the easiest here because it doesn't add another user
space ABI, but I'd strongly recommend not to add any other ones.

       Arnd