linux-kernel - Re: linux-next: build failure after merge of the bpf-next tree

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Message-ID: <CAFULd4aHiEaJkJANNGwv1ae7T0oLd+r9_4+tozgAq0EZhS16Tw@mail.gmail.com>
Date: Wed, 19 Mar 2025 20:43:53 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, Stephen Rothwell <sfr@...b.auug.org.au>, 
	Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, bpf <bpf@...r.kernel.org>, 
	Networking <netdev@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Linux Next Mailing List <linux-next@...r.kernel.org>
Subject: Re: linux-next: build failure after merge of the bpf-next tree

On Wed, Mar 19, 2025 at 7:56 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Mar 19, 2025 at 9:06 AM Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > On Wed, Mar 19, 2025 at 3:55 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Wed, Mar 19, 2025 at 7:36 AM Kumar Kartikeya Dwivedi
> > > <memxor@...il.com> wrote:
> > > >
> > > > > >
> > > > > > I've sent a fix [0], but unfortunately I was unable to reproduce the
> > > > > > problem with an LLVM >= 19 build, idk why. I will try with GCC >= 14
> > > > > > as the patches require to confirm, but based on the error I am 99%
> > > > > > sure it will fix the problem.
> > > > >
> > > > > Probably because __seg_gs has CC_HAS_NAMED_AS depends on CC_IS_GCC.
> > > > > Let me give it a go with GCC.
> > > > >
> > > >
> > > > Can confirm now that this fixes it, I just did a build with GCC 14
> > > > where Uros's __percpu checks kick in.
> > >
> > > Great. Thanks for checking and quick fix.
> > >
> > > btw clang supports it with __attribute__((address_space(256))),
> > > so CC_IS_GCC probably should be relaxed.
> >
> > https://github.com/llvm/llvm-project/issues/93449
> >
> > needs to be fixed first. Also, the feature has to be thoroughly tested
> > (preferably by someone having a deep knowledge of clang) before it is
> > enabled by default.
>
> clang error makes sense to me.

It is not an error, but an internal compiler error. This should never happen.

> What does it even mean to do addr space cast from percpu to normal address:
>
> __typeof__(int __seg_gs) const_pcpu_hot;
> void *__attribute____UNIQUE_ID___addressable_const_pcpu_hot612 =
>     (void *)(long)&const_pcpu_hot;

Please see [1] for an explanation.

[1] https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html#x86-Named-Address-Spaces

Uros.