Date:   Fri, 18 Mar 2022 14:16:58 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>, llvm@...ts.linux.dev,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] kbuild: Make $(LLVM) more flexible

On Fri, Mar 11, 2022 at 2:36 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> 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@...nel.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@...nel.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!


I fixed up ": ::" to "::",
and applied to linux-kbuild.  Thanks.






-- 
Best Regards
Masahiro Yamada

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