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

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Date: Mon, 17 Jun 2024 09:52:51 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Andy Chiu <andy.chiu@...ive.com>
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 Mon, Jun 17, 2024 at 10:38:55AM +0800, Andy Chiu wrote:
> 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

Yes, that definitely makes sense, I forgot that this has been abstracted
away via the use of those alignment Kconfig symbols that can be selected
when needed. Much better than my suggestions, thanks!

> >
> > > +     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