Re: Re: [PATCH v3 1/7] riscv: ftrace: support fastcc in Clang for WITH_ARGS

From: Evgenii Shatokhin <e.shatokhin@yadro.com>
To: "Björn Töpel" <bjorn@kernel.org>
Cc: Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andy Chiu <andybnac@gmail.com>, <linux-kernel@vger.kernel.org>,
	<linux-trace-kernel@vger.kernel.org>,
	<linux-riscv@lists.infradead.org>, <llvm@lists.linux.dev>,
	<bjorn@rivosinc.com>, <puranjay12@gmail.com>,
	<alexghiti@rivosinc.com>, <yongxuan.wang@sifive.com>,
	<greentime.hu@sifive.com>, <nick.hu@sifive.com>,
	<nylon.chen@sifive.com>, <tommy.wu@sifive.com>,
	<eric.lin@sifive.com>, <viccent.chen@sifive.com>,
	<zong.li@sifive.com>, <samuel.holland@sifive.com>
Subject: Re: Re: [PATCH v3 1/7] riscv: ftrace: support fastcc in Clang for WITH_ARGS
Date: Tue, 3 Dec 2024 17:44:25 +0300	[thread overview]
Message-ID: <7e5ce1ce-e30a-46c5-80fb-eb52e59e4dc2@yadro.com> (raw)
In-Reply-To: <87wmghotth.fsf@all.your.base.are.belong.to.us>

Hi,

On 03.12.2024 15:05, Björn Töpel wrote:
> 
> Andy Chiu <andybnac@gmail.com> writes:
> 
>> From: Andy Chiu <andy.chiu@sifive.com>
>>
>> Some caller-saved registers which are not defined as function arguments
>> in the ABI can still be passed as arguments when the kernel is compiled
>> with Clang. As a result, we must save and restore those registers to
>> prevent ftrace from clobbering them.
>>
>> - [1]: https://reviews.llvm.org/D68559
>>
>> Reported-by: Evgenii Shatokhin <e.shatokhin@yadro.com>
>> Closes: https://lore.kernel.org/linux-riscv/7e7c7914-445d-426d-89a0-59a9199c45b1@yadro.com/
>> Acked-by: Nathan Chancellor <nathan@kernel.org>
>> Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
> 
> Fixes tag?

As far as I understand it, Ftrace for RISC-V has had this problem since 
support for FTRACE_WITH_REGS was added. FTRACE_WITH_ARGS inherited it.

So, it should probably be as follows:

Fixes: aea4c671fb98 ("riscv/ftrace: Add DYNAMIC_FTRACE_WITH_REGS support")

It is more of a workaround rather than a fix though, because it is still 
undecided where the problem is, in the kernel or in LLVM/clang. That 
discussion went nowhere, unfortunately, so it is better to use a 
workaround and move on, IMO.

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

Regards,
Evgenii

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