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Message-ID: <20240617165251.GA550301@thelio-3990X>
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
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