linux-kernel - Re: [PATCH v2 2/6] riscv: ftrace: align patchable functions to 4 Byte boundary

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Message-ID: <8734n8ad9n.fsf@all.your.base.are.belong.to.us>
Date: Tue, 13 Aug 2024 13:11:16 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Andy Chiu <andy.chiu@...ive.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
 <aou@...s.berkeley.edu>, Alexandre Ghiti <alexghiti@...osinc.com>, Zong Li
 <zong.li@...ive.com>, Steven Rostedt <rostedt@...dmis.org>, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers
 <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, Justin Stitt
 <justinstitt@...gle.com>, Puranjay Mohan <puranjay@...nel.org>
Cc: Palmer Dabbelt <palmer@...osinc.com>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, Evgenii Shatokhin <e.shatokhin@...ro.com>, Andy Chiu
 <andy.chiu@...ive.com>
Subject: Re: [PATCH v2 2/6] riscv: ftrace: align patchable functions to 4
 Byte boundary

Andy Chiu <andy.chiu@...ive.com> writes:

> We are changing ftrace code patching in order to remove dependency from
> stop_machine() and enable kernel preemption. This requires us to align
> functions entry at a 4-B align address.
>
> However, -falign-functions on older versions of GCC alone was not strong
> enoungh to align all functions. In fact, cold functions are not aligned
> after turning on optimizations. We consider this is a bug in GCC and
> turn off guess-branch-probility as a workaround to align all functions.
>
> GCC bug id: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345
>
> The option -fmin-function-alignment is able to align all functions
> properly on newer versions of gcc. So, we add a cc-option to test if
> the toolchain supports it.
>
> Suggested-by: Evgenii Shatokhin <e.shatokhin@...ro.com>
> Signed-off-by: Andy Chiu <andy.chiu@...ive.com>

Reviewed-by: Björn Töpel <bjorn@...osinc.com>