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Date:   Tue, 14 Apr 2020 13:53:41 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Nick Desaulniers <ndesaulniers@google.com>,
        Ard Biesheuvel <ardb@kernel.org>
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        Stephen Hines <srhines@google.com>,
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        Linus Walleij <linus.walleij@linaro.org>,
        Peter Smith <Peter.Smith@arm.com>,
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        David Howells <dhowells@redhat.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
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        Tejun Heo <tj@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain
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I don't know if this will help, but I feel like folks might be talking
past each other a little here. I see two primary issues that are
colliding, and I just want to call them out specifically...

On Tue, Apr 14, 2020 at 1:59 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> On Mon, 13 Apr 2020 at 22:45, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > 1. continuous integration and randconfigs.  We need CI to help us spot
> > where things are still broken, and help us from regressing the ground
> > we've fought for.  We can't expect kernel developers to test with
> > LLVM.  Currently, we have LLVM builds in numerous kernel continuous

First, is this one. To paraphrase, "we don't want to lose hard-won
ground."

> To be honest with you, I don't actually think iwmmxt is that
> important. But I have already demonstrated how we can use a couple of
> macros to emit the same instructions without resorting to bare
> opcodes, so there is really no need to disable pieces left and right
> because the Clang assembler does not support them outright - it just
> needs someone to care enough about this, rather than rush through the
> list with a tick the box attitude, and rip out the pieces that look a
> bit too complicated.

The second is this one. To paraphrase, "we can just fix things instead
of disabling them."

This feels a lot to me like the pain I (and plenty of others) have felt
when doing treewide changes (adding full compiler support is kind of a
treewide change). The above two points were really strongly felt when we
were trying to remove VLAs. In our case, we didn't even have the option
to disable stuff, so the pain was even worse: VLAs were being added to
the kernel faster than we could remove them.

What's a good middle ground here? For VLAs, I ended up getting akpm's
help by having him add -Wvla to his local builds and send nag emails
to people when they added VLAs. That's not really a thing here, but it
seems like there should be a way to avoid losing ground (in this case,
it's the erosion of attention: repeated known-issue warnings means the
CI gets ignored for the warnings on newly added issues). It does seem
to me like adding the negative depends is a reasonable first step: it
marks what hard things need fixing later without losing coverage for
things that can be more easily fixed now with available resources.

For the specific iwmmxt.S case, perhaps the solution is the suggested
changes? I imagine it should be possible to do a binary diff to see zero
changes before/after.

For others, is a negative depends acceptable? Given how responsive
Nick, Nathan, and others are, I don't think there is any real risk of a
"slippery slope" of things just getting swept under the rug forever.
Everyone here wants to see the kernel be awesome. :)

-Kees

-- 
Kees Cook