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

[Guo Ren <guoren@kernel.org>]

On Wed, Mar 26, 2025 at 2:56 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Wed, Mar 26, 2025, at 07:07, Guo Ren wrote:
> > On Tue, Mar 25, 2025 at 9:18 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >> On Tue, Mar 25, 2025, at 13:26, Peter Zijlstra wrote:
> >> > On Tue, Mar 25, 2025 at 08:15:41AM -0400, guoren@kernel.org wrote:
> >>
> >> 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.
> >
> > I modified uapi with
> > void __user *msg_name;
> > ->
> > union {void __user *msg_name; u64 __msg_name;};
> > to make native 64-bit ABI.
> >
> > I would look at compat stuff instead of using __riscv_xlen macro.
>
> The problem I see here is that there are many more drivers
> that you did not modify than drivers that you did change this
> way.  The union is particularly ugly, but even if you find
> a nicer method of doing this, you now also put the burden
> on future driver writers to do this right for your platform.
Got it.

>
> >> 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.
> >
> > The lp64-abi userspace rootfs works fine in this patch set, which
> > proves the technique is valid. But the modification on uapi is raw,
> > and I'm looking at compat stuff.
>
> There is a big difference between making it work for a particular
> set of userspace binaries and making it correct for the entire
> kernel ABI.
>
> I agree that limiting the hacks to the compat side while keeping
> the native ABI as ilp32 as in your previous versions is better,
> but I also don't think this can be easily done without major
> changes to how compat mode works in general, and that still
> seems like a show-stopper for two reasons:
>
> - it still puts the burden on driver writers to get it right
>   for your platform. The scope is a bit smaller than in the
>   current version because that would be limited to the compat
>   handlers and not change the native codepath, but that's
>   still a lot of drivers.
>
> - the way that I would imagine this to be implemented in
>   practice would require changing the compat code in a way that
>   allows multiple compat ABIs, so drivers can separate the
>   normal 32-on-64 handling from the 64-on-32 version you need.
>   We have discussed something like this in the past, but Linus
>   has already made it very clear that he doesn't want it done
>   that way. Whichever way you do it, this is unlikely to
>   find consensus.
Got it, thanks for analysing.

>
> > Supporting lp64-abi userspace is essential because riscv lp64-abi and
> > ilp32-abi userspace are hybrid deployments when the target is
> > ilp32-abi userspace. The lp64-abi provides a good supplement to
> > ilp32-abi which eases the development.
>
> I'm not following here, please clarify. I do understand that
> having a mixed 32/64 userspace can help for development, but
> that can already be done on a 64-bit kernel and it doesn't
> seem to be useful for deployment because having two sets of
> support libraries makes this counterproductive for the goal
> of saving RAM.
In my case, most binaries and libraries are based on 32-bit, but a
small part would remain on 64-bit, which may be statically linked.
For RISC-V, the rv64 ecosystem is more complete than the rv32's. So,
rv64-abi is always necessary, and rv32-abi is a supplement.

>
> >> 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?
> >
> > Yes, there is, and it only supports rv32 userspace, not rv32 kernel.
> > https://www.xrvm.com/product/xuantie/C908
>
> Ok, thanks for the link.
>
>        Arnd
>


-- 
Best Regards
 Guo Ren