linux-kernel - Re: [RFC PATCH V3 01/43] rv64ilp32_abi: uapi: Reuse lp64 ABI interface

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Message-ID: <CAHk-=wiVgTJpSxrQbEi28pUOmuWXrox45vV9kPhe9q5CcRxEbw@mail.gmail.com>
Date: Tue, 25 Mar 2025 13:41:30 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: guoren@...nel.org
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Subject: Re: [RFC PATCH V3 01/43] rv64ilp32_abi: uapi: Reuse lp64 ABI interface

On Tue, 25 Mar 2025 at 05:17, <guoren@...nel.org> wrote:
>
> The rv64ilp32 abi kernel accommodates the lp64 abi userspace and
> leverages the lp64 abi Linux interface. Hence, unify the
> BITS_PER_LONG = 32 memory layout to match BITS_PER_LONG = 64.

No.

This isn't happening.

You can't do crazy things in the RISC-V code and then expect the rest
of the kernel to just go "ok, we'll do crazy things".

We're not doing crazy __riscv_xlen hackery with random structures
containing 64-bit values that the kernel then only looks at the low 32
bits. That's wrong on *so* many levels.

I'm willing to say "big-endian is dead", but I'm not willing to accept
this kind of crazy hackery.

Not today, not ever.

If you want to run a ilp32 kernel on 64-bit hardware (and support
64-bit ABI just in a 32-bit virtual memory size), I would suggest you

 (a) treat the kernel as natively 32-bit (obviously you can then tell
the compiler to use the rv64 instructions, which I presume you're
already doing - I didn't look)

 (b) look at making the compat stuff do the conversion the "wrong way".

And btw, that (b) implies *not* just ignoring the high bits. If
user-space gives 64-bit pointer, you don't just treat it as a 32-bit
one by dropping the high bits. You add some logic to convert it to an
invalid pointer so that user space gets -EFAULT.

            Linus