From: Eduard Zingerman <eddyz87@gmail.com>
To: Quentin Monnet <quentin@isovalent.com>,
bpf@vger.kernel.org, ast@kernel.org
Cc: andrii@kernel.org, daniel@iogearbox.net, martin.lau@linux.dev,
kernel-team@fb.com, yonghong.song@linux.dev,
alan.maguire@oracle.com
Subject: Re: [RFC v2 2/3] bpftool: add attribute preserve_static_offset for context types
Date: Tue, 12 Dec 2023 17:58:21 +0200 [thread overview]
Message-ID: <90dd9462984be5cfce9db23eda53df44f39a8687.camel@gmail.com> (raw)
In-Reply-To: <baee9fb4-7559-4ba2-a254-7388bb6caa63@isovalent.com>
On Tue, 2023-12-12 at 11:39 +0000, Quentin Monnet wrote:
[...]
> Hi, and thanks for this!
>
> Apologies for missing the discussion on v1. Reading through the previous
> thread I see that they were votes in favour of the hard-coded approach,
> but I would ask folks to please reconsider.
>
> I'm not keen on taking this list in bpftool, it doesn't seem to be the
> right place for that. I understand there's no plan to add new mirror
> context structs, but if we change policy for whatever reason, we're sure
> to miss the update in this list and that's a bug in bpftool. If bpftool
> ever gets ported to Windows and Windows needs support for new structs,
> that's more juggling to do to support multiple platforms. And if any
> tool other than bpftool needs to generate vmlinux.h headers in the
> future, it's back to square one - although by then there might be extra
> pushback for changing the BTF, if bpftool already does the work.
>
> Like Alan, I rather share your own inclination towards the more generic
> declaration tags approach, if you don't mind the additional work.
Understood, thank you for feedback.
The second option is to:
1. Define __bpf_ctx macro in linux headers as follows:
#if __has_attribute(preserve_static_offset) && defined(__bpf__)
#define __bpf_ctx __attribute__((preserve_static_offset)) \
__attribute__((btf_decl_tag(preserve_static_offset)))
#else
#define __bpf_ctx
#endif
2a. Update libbpf to emit __attribute__((preserve_static_offset)) when
corresponding decl tag is present in the BTF.
2b. Update bpftool to emit __attribute__((preserve_static_offset)) for
types with corresponding decl tag. (Like in this patch but check
for decl tag instead of name).
I think that 2b is better, because emitting
BPF_NO_PRESERVE_STATIC_OFFSET from the same place where
BPF_NO_PRESERVE_ACCESS_INDEX makes more sense,
libbpf does not emit any macro definitions at the moment.
wdyt?
next prev parent reply other threads:[~2023-12-12 15:58 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-12 2:31 [RFC v2 0/2] use preserve_static_offset in bpf uapi headers Eduard Zingerman
2023-12-12 2:31 ` [RFC v2 1/3] bpf: Mark virtual BPF context structures as preserve_static_offset Eduard Zingerman
2023-12-12 2:31 ` [RFC v2 2/3] bpftool: add attribute preserve_static_offset for context types Eduard Zingerman
2023-12-12 11:39 ` Quentin Monnet
2023-12-12 15:58 ` Eduard Zingerman [this message]
2023-12-12 16:07 ` Quentin Monnet
2023-12-13 4:53 ` Yonghong Song
2023-12-12 2:31 ` [RFC v2 3/3] selftests/bpf: verify bpftool emits preserve_static_offset Eduard Zingerman
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