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

[Guo Ren]

On Tue, Mar 25, 2025 at 8:27 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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
>
> I'm thinking you're going to be finding a metric ton of assumptions
> about 'unsigned long' being 64bit when 64BIT=y throughout the kernel.
Less than you imagined. Most code is compatible with ILP32 ABI due to
the CONFIG_32BIT. In my practice, it's deemed acceptable.

>
> I know of a couple of places where 64BIT will result in different math
> such that a 32bit 'unsigned long' will trivially overflow.
I would be grateful if you could share some with me.

>
> Please, don't do this. This adds a significant maintenance burden on all
> of us.
The 64ILP32 ABI would bear the maintenance burden, not traditional
64-bit or 32-bit ABIs. The patch set won't impact other CONFIG_64BIT
or CONFIG_32BIT. Numerous RV64 chips require the RV64ILP32 ABI to
reduce the memory and cache footprint; we will bear the burden. The
core code maintainers would receive patches that would make them use
BITS_PER_LONG and CONFIG_64BIT more accurately.

-- 
Best Regards
 Guo Ren