Re: ARCH=arm LLVM_IAS=1 patches for 5.10, 5.4, and 4.19

From: Ard Biesheuvel <ardb@kernel.org>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: "Sasha Levin" <sashal@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"# 3.4.x" <stable@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	"Jian Cai" <jiancai@google.com>, "Stefan Agner" <stefan@agner.ch>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Sami Tolvanen" <samitolvanen@google.com>,
	candle.sun@unisoc.com,
	"Miles Chen (陳民樺)" <miles.chen@mediatek.com>,
	"Stephen Hines" <srhines@google.com>,
	"Luis Lozano" <llozano@google.com>,
	"Sandeep Patil" <sspatil@google.com>,
	"Marc Zyngier" <maz@kernel.org>
Subject: Re: ARCH=arm LLVM_IAS=1 patches for 5.10, 5.4, and 4.19
Date: Mon, 15 Mar 2021 11:37:31 +0100	[thread overview]
Message-ID: <CAMj1kXGLrVXZPAoxTtMueB9toeoktuKza-mRpd4vZ0SLN6bSSQ@mail.gmail.com> (raw)
In-Reply-To: <YE8l2qhycaGPYdNn@kroah.com>

On Mon, 15 Mar 2021 at 10:16, Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Mon, Mar 15, 2021 at 10:12:31AM +0100, Greg KH wrote:
> > On Mon, Mar 15, 2021 at 10:08:49AM +0100, Greg KH wrote:
> > > On Fri, Mar 12, 2021 at 11:07:39PM -0500, Sasha Levin wrote:
> > > > On Fri, Mar 12, 2021 at 09:28:56AM -0800, Nick Desaulniers wrote:
> > > > > My mistake, meant to lop those last two commits off of 4.19.y, they
> > > > > were the ones I referred to earlier working their way through the ARM
> > > > > maintainers tree.  Regenerated the series' (rather than edit the patch
> > > > > files) additionally with --base=auto. Re-attached.
> > > >
> > > > Queued up, thanks!
> > >
> > > This series seems to cause build breakages in a lot of places, so I'm
> > > going to drop the whole set of them now:
> > >     https://lore.kernel.org/r/be846d89-ab5a-f02a-c05e-1cd40acc5baa@roeck-us.net
> > > and:
> > >     https://lore.kernel.org/r/066efc42-0788-8668-2ff5-d431e77068b5@roeck-us.net
> > >
> > > Nick, if you want these merged, can you fix up the errors and resend?
> > >
> > > Perhaps you might want to run these through the tuxbuild tool before
> > > sending?  You should have access to it...
> >
> > Oops, wait, they are fine for 5.10.y, just 4.19 and 5.4 are broken, will
> > go drop those patches only.
>
> Also, these are a lot of churn for 5.4 and 4.19, I'm not convinced it's
> really needed there.  Why again is this required?
>

I think backporting this stuff is causing more problems than it
solves. Note that the 5.4 Thumb2 build is still broken today because
it carries

eff8728fe698 vmlinux.lds.h: Add PGO and AutoFDO input sections

but does not carry

f77ac2e378be ARM: 9030/1: entry: omit FP emulation for UND exceptions
taken in kernel mode

which is tagged as a fix for the former patch, and landed in v5.11.
(Side question: anyone have a clue why the patch in question was never
selected for backporting?)

So I really think we should apply more caution here, and have a
*really* good story on why it is essential that these patches are
backported. In this case, I am not convinced there is one.

-- 
Ard.