Re: [RFC PATCH V3 00/43] rv64ilp32_abi: Build CONFIG_64BIT kernel-self with ILP32 ABI

[Arnd Bergmann]

On Tue, Mar 25, 2025, at 13:26, Peter Zijlstra wrote:
> On Tue, Mar 25, 2025 at 08:15:41AM -0400, guoren@xxxxxxxxxx wrote:
>> From: "Guo Ren (Alibaba DAMO Academy)" <guoren@xxxxxxxxxx>
>> 
>> Since 2001, the CONFIG_64BIT kernel has been built with the LP64 ABI,
>> but this patchset allows the CONFIG_64BIT kernel to use an ILP32 ABI
>
> Please, don't do this. This adds a significant maintenance burden on all
> of us.

It would be easier to this with CONFIG_64BIT disabled and continue
treating CONFIG_64BIT to be the same as BITS_PER_LONG=64, but I still
think it's fundamentally a bad idea to support this in mainline
kernels in any variation, other than supporting regular 32-bit
compat mode tasks on a regular 64-bit kernel.

>> The patchset targets RISC-V and is built on the RV64ILP32 ABI, which
>> was introduced into RISC-V's psABI in January 2025 [1]. This patchset
>> equips an rv64ilp32-abi kernel with all the functionalities of a
>> traditional lp64-abi kernel, yet restricts the address space to 2GiB.
>> Hence, the rv64ilp32-abi kernel simultaneously supports lp64-abi
>> userspace and ilp32-abi (compat) userspace, the same as the
>> traditional lp64-abi kernel.

You declare the syscall ABI to be the native 64-bit ABI, but this
is fundamentally not true because a many uapi structures are
defined in terms of 'long' or pointer values, in particular in
the ioctl call. This might work for an rv64ilp32 userspace that
uses the same headers and the same types, but you explicitly
say that the goal is to run native rv64 or compat rv32 tasks,
not rv64ilp32 (thanks!).

As far as I can tell, there is no way to rectify this design flaw
other than to drop support for 64-bit userspace and only support
regular rv32 userspace. I'm also skeptical that supporting rv64
userspace helps in practice other than for testing, since
generally most memory overhead is in userspace rather than the
kernel, and there is much more to gain from shrinking the larger
userspace by running rv32 compat mode binaries on a 64-bit kernel
than the other way round.

If you remove the CONFIG_64BIT changes that Peter mentioned and
the support for ilp64 userland from your series, you end up
with a kernel that is very similar to a native rv32 kernel
but executes as rv64ilp32 and runs rv32 userspace. I don't have
any objections to that approach, and the same thing has come
up on arm64 as a possible idea as well, but I don't know if
that actually brings any notable advantage over an rv32 kernel.

Are there CPUs that can run rv64 kernels and rv32 userspace
but not rv32 kernels, similar to what we have on Arm Cortex-A76
and Cortex-A510?

       Arnd