linux-kernel - Re: [PATCH 4/8] riscv: ftrace: align patchable functions to 4 Byte boundary

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Date: Mon, 17 Jun 2024 10:38:55 +0800
From: Andy Chiu <andy.chiu@...ive.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: 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>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Puranjay Mohan <puranjay@...nel.org>, 
	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>
Subject: Re: [PATCH 4/8] riscv: ftrace: align patchable functions to 4 Byte boundary

On Fri, Jun 14, 2024 at 3:09 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Hi Andy,
>
> On Thu, Jun 13, 2024 at 03:11:09PM +0800, Andy Chiu wrote:
> > 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>
> > ---
> >  arch/riscv/Kconfig  | 1 +
> >  arch/riscv/Makefile | 7 ++++++-
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index b94176e25be1..80b8d48e1e46 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -203,6 +203,7 @@ config CLANG_SUPPORTS_DYNAMIC_FTRACE
> >  config GCC_SUPPORTS_DYNAMIC_FTRACE
> >       def_bool CC_IS_GCC
> >       depends on $(cc-option,-fpatchable-function-entry=8)
> > +     depends on $(cc-option,-fmin-function-alignment=4) || !RISCV_ISA_C
>
> Please use CC_HAS_MIN_FUNCTION_ALIGNMENT (from arch/Kconfig), which
> already checks for support for this option.

Thanks for the suggestion!

>
> >  config HAVE_SHADOW_CALL_STACK
> >       def_bool $(cc-option,-fsanitize=shadow-call-stack)
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index 06de9d365088..74628ad8dcf8 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -14,8 +14,13 @@ endif
> >  ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
> >       LDFLAGS_vmlinux += --no-relax
> >       KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
> > +ifeq ($(CONFIG_CC_IS_CLANG),y)
>
> Same here, please invert this and use
>
>   ifdef CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT
>
> like the main Makefile does.

Hope this makes sense to you. I am going to add the following in riscv Kconig:

select FUNCTION_ALIGNMENT_4B if DYNAMIC_FTRACE && !RISCV_ISA_C

So we will not need any of these

>
> > +     cflags_ftrace_align := -falign-functions=4
> > +else
> > +     cflags_ftrace_align := -fmin-function-alignment=4
> > +endif
> >  ifeq ($(CONFIG_RISCV_ISA_C),y)
> > -     CC_FLAGS_FTRACE := -fpatchable-function-entry=4
> > +     CC_FLAGS_FTRACE := -fpatchable-function-entry=4 $(cflags_ftrace_align)
> >  else
> >       CC_FLAGS_FTRACE := -fpatchable-function-entry=2
> >  endif
> >
> > --
> > 2.43.0
> >
> >

Thanks,
Andy