Re: [PATCH] Makefile: add comment to discourage tools/* addition for kernel builds

Masahiro Yamada
2024-06-26T19:03:24Z

On Fri, Jun 21, 2024 at 6:52 AM Brian Norris <briannorris@chromium.org> wrote:
>
> On Wed, Jun 19, 2024 at 03:21:42PM +0900, Masahiro Yamada wrote:
> > Kbuild provides scripts/Makefile.host to build host programs used for
> > building the kernel. Unfortunately, there are two exceptions that opt
> > out of Kbuild. The build system under tools/ is a cheesy replica, and
> > is always a disaster. I was recently poked about a problem in the tools
> > build issue, which I do not maintain (and nobody maintains). [1]
>
> (Side note: I hope I haven't placed undue burden on you; I understood
> you don't maintain tools/ and that it didn't use Kbuild. I only "poked"
> you because the original bug report I was replying to had you and
> linux-kbuild on CC already. And I appreciate your engagement, even if
> the bugs are due to intentional forking.)

I did not mean to express my complaint particularly with the previous thread.

It is not the first time that the tools/ build issue arose.

I will drop the references to the threads.

> But anyway, I agree that clearer documentation and recommendations could
> be helpful here. To that end, some dumb questions below, as I'm not sure
> if this fully serves its purpose as-is:
>
> > Without a comment, somebody might believe this is the right location
> > because that is where objtool lives, even when a more robust Kbuild
> > syntax satisfies their needs. [2]
> >
> > [1]: https://lore.kernel.org/linux-kbuild/ZnIYWBgrJ-IJtqK8@google.com/T/#m8ece130dd0e23c6f2395ed89070161948dee8457
> > [2]: https://lore.kernel.org/all/20240618200501.GA1611012@google.com/
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> > ---
> >
> > Makefile | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/Makefile b/Makefile
> > index 471f2df86422..ba070596ad4e 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1331,6 +1331,11 @@ prepare: tools/bpf/resolve_btfids
> > endif
> > endif
> >
> > +# README
> > +# The tools build system is not a part of Kbuild. Before adding yet another
> > +# tools/* here, please consider if the standard "hostprogs" syntax satisfies
> > +# your needs.
> > +
>
> Some clarifying questions / statements-as-questions:
>
> * nothing in tools/ uses Kbuild, right? (even stuff that uses KBUILD_*
> names is just an imitative port, right?)

Correct.

You can build a tool from multiple directory locations.

For example, you can compile the 'perf' in multiple locations.

[1] From the top of the kernel tree

$ make tools/perf

[2] From the tools/ directory

$ cd tools
$ make perf

[3] From the tools/perf/ directory

$ cd tools/perf
$ make

When you do [2] or [3], the top-level Makefile is not parsed.

If necessary, the tools build system copies code from Kbuild.

> * not everything in tools/ is actually promoted to a high-level target,
> that affects this top-level Makefile. Are you only concerned about
> stuff that pretends to be integrated in the top-level kernel Makefile?
> (If not, then it seems like placing the README comments only in this
> Makefile is a poor choice.)

The tool build is integrated as a pattern rule in the top Makefile.
(tools/%)

So, you can build other tools from the top Makefile.

See commit ea01fa9f63aef, which did not get Ack from any Kbuild
maintainer, and caused subsequent troubles, and the benefit
of which I still do not understand.

Supporting "make tools/perf" in addition to "make -C tools perf"
only saved a few characters to type.

So, the problem remains, unless I revert ea01fa9f63aef.

I decided to not care about it too much, as long as
such tools are not used during the kernel build.

I am really worried about objtool and resolve_btfids,
as these two are used for building the kernel.

> * is the "standard hostprogs" recommendation a general recommendation,
> for all sorts of kept-in-the-kernel-tree host tools? Is the
> recommendation to "use Kbuild" or to "avoid putting your tool in
> tools/*"? Is it possible (recommended?) to plumb Kbuild stuff into
> tools/, even if other parts won't migrate?

I do not know.

They are different build systems with different designs.

Kbuild always works in the top of the output directory.
Kbuild changes the working directory at most once if O= is given,
but otherwise, it never changes the working directory during the build.

The tools/ build system changes the working directory every time
it invokes a new Make, and compiles the tool in its source directory.

I do not know if all tools want to Kbuild.
(the same applied to kselftest)

I think I can convert objtool and resolve_btfids to the Kbuild way.

>
> As is, I can't tell if this is telling people to avoid adding new stuff
> to tools/ entirely, or just to only add to tools/ if you're able to
> remain completely isolated from the rest of the kernel build -- as soon
> as you want to play some part in the Kbuild-covered part of the tree,
> you need to use Kbuild.

See the code in the top Makefile.

'prepare' depends on tools/objtool and tools/bpf/resolve_btfids.

If other tools are not prerequisites of 'scripts',
Kbuild will not compile them.

>
> If I'm inferring the right answers to the above, then maybe an improved
> wording could be something like:
>
> "The tools build system is not a part of Kbuild and tends to introduce
> its own unique issues. If you need to integrate a new tool into Kbuild,
> please consider locating that tool outside the tools/ tree and using the
> standard Kbuild "hostprogs" syntax instead of adding a new tools/* entry
> here."

I am fine with this description.

Nicolas suggested a link to Documentation/kbuild/makefiles.rst

We can combine the two.

# The tools build system is not a part of Kbuild and tends to introduce
# its own unique issues. If you need to integrate a new tool into Kbuild,
# please consider locating that tool outside the tools/ tree and using the
# standard Kbuild "hostprogs" syntax instead of adding a new tools/* entry
# here. See Documentation/kbuild/makefiles.rst for details.

> It's possible I'm playing mental acrobatics here in my reading too.
>
> Either way, I think this is a good trajectory:
>
> Reviewed-by: Brian Norris <briannorris@chromium.org>
>
> Regards,
> Brian
>
> > PHONY += resolve_btfids_clean
> >
> > resolve_btfids_O = $(abspath $(objtree))/tools/bpf/resolve_btfids
> > --
> > 2.43.0
> >

--
Best Regards

Masahiro Yamada