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Date:   Thu, 18 May 2023 17:14:36 -0700 (PDT)
From:   Paul Walmsley <paul.walmsley@sifive.com>
To:     Palmer Dabbelt <palmer@rivosinc.com>
cc:     guoren@kernel.org, Arnd Bergmann <arnd@arndb.de>,
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Subject: Re: [RFC PATCH 00/22] riscv: s64ilp32: Running 32-bit Linux kernel
 on 64-bit supervisor mode
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On Thu, 18 May 2023, Palmer Dabbelt wrote:

> On Thu, 18 May 2023 06:09:51 PDT (-0700), guoren@kernel.org wrote:
>
> > This patch series adds s64ilp32 support to riscv. The term s64ilp32
> > means smode-xlen=64 and -mabi=ilp32 (ints, longs, and pointers are all
> > 32-bit), i.e., running 32-bit Linux kernel on pure 64-bit supervisor
> > mode. There have been many 64ilp32 abis existing, such as mips-n32 [1],
> > arm-aarch64ilp32 [2], and x86-x32 [3], but they are all about userspace.
> > Thus, this should be the first time running a 32-bit Linux kernel with
> > the 64ilp32 ABI at supervisor mode (If not, correct me).
> 
> Does anyone actually want this?  At a bare minimum we'd need to add it to the
> psABI, which would presumably also be required on the compiler side of things.
> 
> It's not even clear anyone wants rv64/ilp32 in userspace, the kernel seems
> like it'd be even less widely used.

We've certainly talked to folks who are interested in RV64 ILP32 userspace 
with an LP64 kernel.  The motivation is the usual one: to reduce data size 
and therefore (ideally) BOM cost.  I think this work, if it goes forward, 
would need to go hand in hand with the RVIA psABI group.

The RV64 ILP32 kernel and ILP32 userspace approach implemented by this 
patch is intriguing, but I guess for me, the question is whether it's 
worth the extra hassle vs. a pure RV32 kernel & userspace.  


- Paul