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SubjectRe: [PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM
On 2020-04-06, Masahiro Yamada wrote:
>On Sat, Apr 4, 2020 at 3:24 AM Nick Desaulniers <> wrote:
>> On Thu, Apr 2, 2020 at 10:17 PM Masahiro Yamada <> wrote:
>> >
>> > As Documentation/kbuild/llvm.rst implies, building the kernel with a
>> > full set of LLVM tools gets very verbose and unwieldy.
>> >
>> > Provide a single switch 'LLVM' to use Clang and LLVM tools instead of
>> > GCC and Binutils. You can pass LLVM=1 from the command line or as an
>> > environment variable. Then, Kbuild will use LLVM toolchains in your
>> > PATH environment.
>> >
>> > Please note LLVM=1 does not turn on the LLVM integrated assembler.
>> > You need to explicitly pass AS=clang to use it. When the upstream
>> > kernel is ready for the integrated assembler, I think we can make
>> > it default.
>> Having this behavior change over time may be surprising. I'd rather
>> that if you want to not use the integrated assembler, you explicitly
>> negate it, or just don't use the LLVM=1 syntax, ie. `make CC=clang
>> LD=ld.lld ...`.
>> We could modify how `-no-integrated-as` is chosen when LLVM=1.
>> make LLVM=1 LLVMIA=0 ... # add `-no-integrated-as`
>> # what the flag is doesn't really matter to me, something shorter might be nice.
>> make LLVM=1 # use all LLVM tools
>> Since we got rid of $(AS), it would be appropriate to remove/change it
>> there, since no one really relies on AS=clang right now. (We do have 1
>> of our 60+ CI targets using it, but we can also change that trivially.
>> So I think we have a lot of freedom to change how `-no-integrated-as`
>> is set.
>> This could even be independent of this patch.
>I also thought a boolean flag is preferred.
>AS=clang will not live long anyway, and
>I hesitated to break the compatibility
>for the short-term workaround.
>But, if this is not a big deal, I can
>replace AS=clang with LLVMIA=1.

My mere complaint is that it may be difficult to infer the intention (integrated
assembler) from the abbreviation "IA" in "LLVMIA" :/

Something with "AS" in the name may be easier for a user to understand,

>> >
>> > We discussed what we need, and we agreed to go with a simple boolean
>> > switch (
>> >
>> > Some items in the discussion:
>> >
>> > - LLVM_DIR
>> >
>> > When multiple versions of LLVM are installed, I just thought supporting
>> > LLVM_DIR=/path/to/my/llvm/bin/ might be useful.
>> >
>> > CC = $(LLVM_DIR)clang
>> > LD = $(LLVM_DIR)ld.lld
>> > ...
>> >
>> > However, we can handle this by modifying PATH. So, we decided to not do
>> > this.
>> >
>> >
>> > Some distributions (e.g. Debian) package specific versions of LLVM with
>> > naming conventions that use the version as a suffix.
>> >
>> > CC = clang$(LLVM_SUFFIX)
>> > LD = ld.lld(LLVM_SUFFIX)
>> > ...
>> >
>> > will allow a user to pass LLVM_SUFFIX=-11 to use clang-11 etc.,
>> > but the suffixed versions in /usr/bin/ are symlinks to binaries in
>> > /usr/lib/llvm-#/bin/, so this can also be handled by PATH.
>> >
>> > - HOSTCC, HOSTCXX, etc.
>> >
>> > We can switch the host compilers in the same way:
>> >
>> > ifneq ($(LLVM),)
>> > HOSTCC = clang
>> > HOSTCXX = clang++
>> > else
>> > HOSTCC = gcc
>> > HOSTCXX = g++
>> > endif
>> >
>> > This may the right thing to do, but I could not make up my mind.
>> > Because we do not frequently switch the host compiler, a counter
>> > solution I had in my mind was to leave it to the default of the
>> > system.
>> >
>> > HOSTCC = cc
>> > HOSTCXX = c++
>> >
>> > Many distributions support update-alternatives to switch the default
>> > to GCC, Clang, or whatever, but reviewers were opposed to this
>> > approach. So, this commit does not touch the host tools.
>> update-alternatives assumes you've installed Clang via a package manager?
>> $ update-alternatives --list cc
>> /usr/bin/gcc
>> On my system even though clang and friends are in my PATH.
>> And previously, there was feedback that maybe folks don't want to
>> change `cc` on their systems just for Clang kernel builds.
>> A goal for ClangBuiltLinux is to build a kernel image with no GCC or
>> binutils installed on the host. Let the record reflect that. And
>> there's been multiple complaints that the existing syntax is too long
>> for specifying all of the tools.
>> LLVM=1 is meant to be one flag. Not `make LLVM=1 HOSTCC=clang
>> HOSTCXX=clang`. If folks want fine grain flexibility, use the
>> existing command line interface, which this patch does not change.
>> LLVM=1 is opinionated, and inflexible, because it makes a strong
>> choice to enable LLVM for everything.
>> Another reason why I don't want to change these over time, and why I
>> want them all to be in sync is that there are 4 different CI systems
>> for the kernel, and they are currently fragmented in terms of who is
>> using what tools:
>> KernelCI: CC=clang only
>> Kbuild test robot aka 0day bot: CC=clang LD=ld.lld
>> Linaro TCWG: CC=clang only
>> our CI: a complete mix due to combinatorial explosion, but more
>> coverage of LLVM than everyone else.
>> That is a mess that we must solve. Having 1 flag that works
>> consistently across systems is one solution. Now if those were all
>> using LLVM=1, but some were enabling Clang's integrated assembler, and
>> some weren't because we changed the default over time, then we'd be
>> right back to this mismatch between systems. I'd much rather draw the
>> line in the sand, and say "this is how this flag will work, since day
>> 1." Maybe it's too rigid, but it's important to me that if we create
>> something new to solve multiple objectives (1. simplifies existing
>> interface. 2. turns on everything.) that it does so. It is a partial
>> solution, if it eliminates some of the flags while leaving others. I
>> want a full solution.
>> If folks want the flexibility to mix and match tools, the existing
>> interface is capable. But for us to track who is using what, we need
>> 1 flag that we know is not different depending on the cc of the
>> system. Once clang's integrated assembler is good to go, we will
>> begin recommending LLVM=1 to everyone. And we want feedback if we
>> regress building the host utilities during a kernel build, even if
>> there are not many.
>> I'm on the fence about having all of the above satisfied by one patch,
>> or taking this patch as is and following up on the above two points
>> (related to disabling `-no-integrated-as` and setting HOSTCC). I
>> trust your judgement and respect your decisions, so I'll defer to you
>> Masahiro, but I need to make explicit the design goals. Maybe with
>> this additional context it can help inform the design.
>> Tested-by: Nick Desaulniers <>
>Thanks for the comments.
>I'd rather want to do this incrementally,
>making sure I am doing right.
>The meaning of LLVM=1 may change over time.
>It means "the recommended settings" at the moment.
>If CI does not want to change the behavior across
>kernel versions, it can pass individual variables
>Best Regards
>Masahiro Yamada
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