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Date:   Tue, 18 Aug 2020 22:32:41 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 0/9] Add support for type-based and enum
 value-based CO-RE relocations

On Tue, Aug 18, 2020 at 6:37 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Aug 18, 2020 at 06:31:51PM -0700, Andrii Nakryiko wrote:
> > On Tue, Aug 18, 2020 at 6:21 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Tue, Aug 18, 2020 at 03:39:12PM -0700, Andrii Nakryiko wrote:
> > > > This patch set adds libbpf support to two new classes of CO-RE relocations:
> > > > type-based (TYPE_EXISTS/TYPE_SIZE/TYPE_ID_LOCAL/TYPE_ID_TARGET) and enum
> > > > value-vased (ENUMVAL_EXISTS/ENUMVAL_VALUE):
> > > >
> > > > LLVM patches adding these relocation in Clang:
> > > >   - __builtin_btf_type_id() ([0], [1], [2]);
> > > >   - __builtin_preserve_type_info(), __builtin_preserve_enum_value() ([3], [4]).
> > >
> > > I've applied patches 1-4, since they're somewhat indepedent of new features in 5+.
> > > What should be the process to land the rest?
> > > Land llvm first and add to bpf/README.rst that certain llvm commmits are necessary
> > > to build the tests?
> >
> > Clang patches landed about two weeks ago, so they are already in Clang
> > nightly builds. libbpf CI should work fine as it uses clang-12 nightly
> > builds.
> >
> >
> > > But CI will start failing. We can wait for that to be fixed,
> > > but I wonder is there way to detect new clang __builtins automatically in
> > > selftests and skip them if clang is too old?
> >
> > There is a way to detect built-ins availability (__has_builtin macro,
> > [0]) from C code. If we want to do it from Makefile, though, we'd need
> > to do feature detection similar to how we did reallocarray and
> > libbpf-elf-mmap detection I just removed in the other patch set :).
> > Then we'll also need to somehow blacklist tests. Maintaining that
> > would be a pain, honestly. So far selftests/bpf assumed the latest
> > Clang, though, so do you think we should change that, or you were
> > worried that patches hadn't landed yet?
>
> I was hoping that libbpf.h can have builtins unconditionally, but selftests can
> do feature detection automatically and mark them as 'skip'.
> People have been forever complaining about constant need to upgrade clang.
> In this case I think the feature is not fundamental enough (unlike the first
> set of builtins) to force adoption of new clang.
> If/when we start using these new builtins beyond selftests
> that would be a different story.

Ok, took some tinkering to do this for btf_type_id() tests, but
everything works now as you described above. Posted v2.

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