[RFC PATCH 00/22] riscv: s64ilp32: Running 32-bit Linux kernel on 64-bit supervisor mode

[Arnd Bergmann]

On Fri, May 19, 2023, at 02:38, Paul Walmsley wrote:
> On Thu, 18 May 2023, Arnd Bergmann wrote:
>
>> We have had long discussions about supporting ilp32 userspace on
>> arm64, and I think almost everyone is glad we never merged it into
>> the mainline kernel, so we don't have to worry about supporting it
>> in the future. The cost of supporting an extra user space ABI
>> is huge, and I'm sure you don't want to go there. The other two
>> cited examples (mips-n32 and x86-x32) are pretty much unused now
>> as well, but still have a maintenance burden until they can finally
>> get removed.
>
> There probably hasn't been much pressure to support Aarch64 ILP32 since 
> ARM still has hardware support for Aarch32.  Will be interesting to see if 
> that's still the case after ARM drops Aarch32 support for future designs.

I think there was a some pressure for 64ilp32 from Arm when aarch64 support
was originally added, as they always planned to drop aarch32 support
eventually, but I don't see that coming back now. I think the situation
is quite different as well:

On aarch64, there is a significant cost in supporting aarch32 userspace
because of the complexity of that particular instruction set, but at
the same time there is also a huge amount of software that is compiled
for or written to support aarch32 software, and nobody wants to
replace that.  There are also a lot of existing arm32 chips with
guaranteed availability well into the 2030s, new 32-bit-only chips
based on Cortex-A7 (originally released in 2011) coming out constantly,
and even the latest low-end core (Cortex-A510 r1). It's probably
going to be several years before that core even shows up in low-memory
systems, and then decades before this stops being available in SoCs,
even in the unlikely case that no future low-end cores support
aarch32-el0 mode (it's already been announced that there are no
plans for future high-end cores with aarch32 mode, but those won't
be used in low-memory configurations anyway).

For RISC-V, I have not seen much interest in Linux userspace for
the existing rv32 mode, so you could argue that there is not much
to lose in abandoning it. On the other hand, the cost of adding
rv32 support to an rv64 core should be very small as all the
instructions are already present in some other encoding, and
developers have already spent a significant amount of work on
bringing up rv32 userspace that would all have to be done again
for a new ABI, and you'd end up splitting the already tiny
developer base for 32-bit riscv in two for the existing rv32 side
and a new rv64ilp32 side. 

I suppose the answer in both cases is the same though: if a
SoC maker wants to sell a product to users with low memory,
they should pick a CPU core that implements standard 32-bit
user space support rather than making a mess of it and
expecting software to work around it.

      Arnd