linux-kernel - Re: [PATCH v2] kbuild: Use -fmin-function-alignment when available

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Date: Wed, 21 Feb 2024 10:50:59 +0000
From: Mark Rutland <mark.rutland@....com>
To: Petr Pavlu <petr.pavlu@...e.com>
Cc: masahiroy@...nel.org, nathan@...nel.org, nicolas@...sle.eu,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kbuild: Use -fmin-function-alignment when available

On Tue, Feb 20, 2024 at 04:28:23PM +0100, Petr Pavlu wrote:
> On 2/19/24 18:20, Mark Rutland wrote:
> > On Thu, Feb 15, 2024 at 04:16:42PM +0100, Petr Pavlu wrote:
> >> GCC recently added option -fmin-function-alignment, which should appear
> >> in GCC 14. Unlike -falign-functions, this option causes all functions to
> >> be aligned at the specified value, including the cold ones.
> >>
> >> Detect availability of -fmin-function-alignment and use it instead of
> >> -falign-functions when present. Introduce CC_HAS_SANE_FUNCTION_ALIGNMENT
> >> and make the workarounds for the broken function alignment conditional
> >> on this setting.
> >>
> >> Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
> > 
> > I don't have a GCC 14 build to play with, but this looks sound to me.
> > 
> > Petr, are you able to test an arm64 kernel with this and DYNAMIC_FTRACE
> > enabled? i.e. build that, and check that function symbols are all aligned to 8
> > bytes using objdump or similar? That way we could be pretty sure there's no
> > other latent issue in this area.
> 
> I tested an arm64 kernel with DYNAMIC_FTRACE, in particular with
> DYNAMIC_FTRACE_WITH_CALL_OPS=y. That is actually the primary motivation
> for this patch. We ran in our environment into some incorrectly aligned
> functions with this option despite the kernel workarounds. They were
> reported as "Misaligned patch-site" warnings from ftrace_call_adjust().
> I don't observe them anymore with -fmin-function-alignment in my tests.
> Sorry, I should have mentioned this motivation in the commit message.

No problem; thanks for confirming!

Mark.