Re: [PATCH] arm64: disable patchable function entry on big-endian clang builds

[Fangrui Song]

On 2020-05-06, Nathan Chancellor wrote:
> On Wed, May 06, 2020 at 12:22:58PM +0200, Arnd Bergmann wrote:
> > On Wed, May 6, 2020 at 5:45 AM Nathan Chancellor
> > <natechancellor@xxxxxxxxx> wrote:
> > > On Tue, May 05, 2020 at 07:42:43PM +0200, Torsten Duwe wrote:
> > > > On Tue, 5 May 2020 15:25:56 +0100 Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > > > On Tue, May 05, 2020 at 04:12:36PM +0200, Arnd Bergmann wrote:
> > > > > This practically rules out a BE distro kernel with things like PAC,
> > > > > which isn't ideal.
> > >
> > > To be fair, are there big endian AArch64 distros?
> > >
> > > https://wiki.debian.org/Arm64Port: "There is also a big-endian version
> > > of the architecture/ABI: aarch64_be-linux-gnu but we're not supporting
> > > that in Debian (so there is no corresponding Debian architecture name)
> > > and hopefully will never have to. Nevertheless you might want to check
> > > for this by way of completeness in upstream code."
> > >
> > > OpenSUSE and Fedora don't appear to have support for big endian.
> >
> > I don't think any of the binary distros ship big endian ARM64. There are
> > a couple of users that tend to build everything from source using Yocto
> > or similar embedded distros, but as far as I can tell this is getting less
> > common over time as applications get ported to be compatible with
> > big-endian, or get phased out and replaced by code running on regular
> > little-endian systems.
> >
> > The users we see today are likely in telco, military or aerospace
> > settings (While earth is mostly little-endian these days, space is
> > definitely big-endian) that got ported from big-endian hardware, but
> > often with a high degree of customization and long service life.
>
> Ah yes, that makes sense, thanks for the information and background.
> Helps orient myself for the future.
>
> > My policy for Arm specific kernel code submissions is generally that
> > it should be written so it can work on either big-endian or little-endian
> > using the available abstractions (just like any architecture independent
> > code), but I don't normally expect it to be tested on big endian.
> >
> > There are some important examples of code that just doesn't work
> > on big-endian because it's far too hard, e.g. the UEFI runtime services.
> > That is also ok, if anyone really needs it, they can do the work.
> >
> > > > I suggest to get a quote from clang folks first about their schedule and
> > > > regarded importance of patchable-function-entries on BE, and leave it as
> > > > is: broken on certain clang configurations. It's not the kernel's fault.
> > >
> > > We can file an upstream PR (https://bugs.llvm.org) to talk about this
> > > (although I've CC'd Fangrui) but you would rather the kernel fail to
> > > work properly than prevent the user from being able to select that
> > > option? Why even have the "select" or "depends on" keyword then?

Created https://reviews.llvm.org/D79495 to allow the function attribute
'patchable_function_entry' on aarch64_be.
I think -fpatchable-function-entry= just works.

Note, LLD does not support aarch64_be
(https://github.com/ClangBuiltLinux/linux/issues/380).

> > I definitely want all randconfig kernels to build without warnings,
> > and I agree with you that making it just fail at build time is not
> > a good solution.
> >
> > > That said, I do think we should hold off on this patch until we hear
> > > from the LLVM developers.
> >
> > +1
> >
> >       Arnd
>
> Glad we are on the same page.
>
> Cheers,
> Nathan