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Date:   Thu, 10 Mar 2022 10:36:43 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        Shuah Khan <shuah@kernel.org>, llvm@lists.linux.dev,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2] kbuild: Make $(LLVM) more flexible
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On Wed, Mar 09, 2022 at 06:33:40PM +0900, Masahiro Yamada wrote:
> On Wed, Mar 9, 2022 at 12:47 AM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > On Mon, Mar 07, 2022 at 11:08:29AM -0800, Nick Desaulniers wrote:
> > > On Fri, Mar 4, 2022 at 9:14 AM Nathan Chancellor <nathan@kernel.org> wrote:
> > > >
> > > > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> > > > index d32616891dcf..68b74416ec48 100644
> > > > --- a/Documentation/kbuild/llvm.rst
> > > > +++ b/Documentation/kbuild/llvm.rst
> > > > @@ -49,17 +49,36 @@ example: ::
> > > >  LLVM Utilities
> > > >  --------------
> > > >
> > > > -LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
> > > > -to enable them. ::
> > > > -
> > > > -       make LLVM=1
> > > > -
> > > > -They can be enabled individually. The full list of the parameters: ::
> > > > +LLVM has substitutes for GNU binutils utilities. They can be enabled individually.
> > > > +The full list of supported make variables: ::
> > > >
> > > >         make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
> > > >           OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
> > > >           HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
> > > >
> > > > +To simplify the above command, Kbuild supports the ``LLVM`` variable: ::
> > > > +
> > > > +       make LLVM=1
> > > > +
> > > > +If your LLVM tools are not available in your PATH, you can supply their
> > > > +location using the LLVM variable with a trailing slash: ::
> > > > +
> > > > +       make LLVM=/path/to/llvm/
> > > > +
> > > > +which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc.
> > >
> > > I don't think we should do this; `PATH=/path/to/llvm/ make LLVM=1`
> > > works and (my interpretation of what) Masahiro said "if anyone asks
> > > for this, here's how we could do that."  I don't think I've seen an
> > > explicit ask for that. I'd rather LLVM= have 2 behaviors than 3, but I
> > > won't hold this patch up over that.  Either way:
> >
> > Right, there has not been an explicit ask for the prefix support yet,
> > although I know I personally would use it, but I think that it is worth
> > doing now instead of later for a few reasons:
> >
> > 1. It makes path goofs easier to spot. If you do
> >
> >      $ PATH=/path/to/llvm:$PATH make LLVM=1 ...
> >
> >    with a path to LLVM that does not exist (maybe you are bisecting an
> >    issue and using a temporary build of LLVM and you forgot the path it
> >    was in), you fall back to the LLVM tools that are in other places in
> >    your PATH, which is not what the developer intended. I know that I
> >    have messed up bisects that way. If you did
> >
> >      $ make LLVM=/path/to/llvm/
> >
> >    with a path that does not exist, there will be an error much earlier:
> >
> >      $ make LLVM=/this/path/does/not/exist/ defconfig
> >      /bin/sh: line 1: /this/path/does/not/exist/clang: No such file or directory
> >
> > 2. It does not take that much more code or documentation to support. It
> >    is the same amount of code as the suffix and the documentation is
> >    roughly the same amount of lines as well.
> >
> > 3. If we wait to implement the path-based use of $(LLVM), we have three
> >    "sequence" points: the initial support of $(LLVM), the suffix
> >    support, and the prefix support. As we are constantly working with
> >    various trees, it would make it harder to know what to use when. If
> >    we just do it in the same patch, we know 5.18+ can use both of these
> >    methods.
> >
> > However, at the end of the day, we are a team and if you feel like we
> > should only have suffix support, I am more than happy to push a v3 that
> > does just that and we can revist prefix support in the future. Just let
> > me know!
> 
> 
> I do not have a strong opinion about this.
> (I just mentioned the LLVM=/path/to/llvm/ form because I guessed
> somebody would request this sooner or later.)
> 
> 
> If you want me to pick up this version, I will apply it with fixing up
> a nit pointed out by Kees   (": ::" -> "::")
> 
> If you want to send v3, that is fine with me as well.
> 
> Please let me know your thoughts.

Given Nick's response, please pick up this revision with Kees' nit.
Thank you!

Cheers,
Nathan

> > > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > >
> > > > +
> > > > +If your LLVM tools have a version suffix and you want to test with that
> > > > +explicit version rather than the unsuffixed executables like ``LLVM=1``, you
> > > > +can pass the suffix using the ``LLVM`` variable: ::
> > > > +
> > > > +       make LLVM=-14
> > > > +
> > > > +which will use ``clang-14``, ``ld.lld-14``, etc.
> > > > +
> > > > +``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like
> > > > +``LLVM=1``.
> > >
> > > Hmm... I can see someone's build wrappers setting LLVM=1, then them
> > > being surprised that appending LLVM=0 doesn't disable LLVM=1 as they
> > > might expect.  But Masahiro says let's fix this later which is fine.
> >
> > Sure, I guess that is a reasonable case to support. I'll see if I can
> > come up with something that makes sense after this change lands.
> >
> > Cheers,
> > Nathan
> 
> 
> 
> -- 
> Best Regards
> Masahiro Yamada