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Subject: Re: [PATCH 1/9] LoongArch/ftrace: Add basic support
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Huacai Chen <chenhuacai@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        WANG Xuerui <kernel@xen0n.name>, loongarch@lists.linux.dev,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Jiaxun Yang <jiaxun.yang@flygoat.com>, hejinyang@loongson.cn
References: <20220819081403.7143-1-zhangqing@loongson.cn>
 <20220819081403.7143-2-zhangqing@loongson.cn>
 <20220819132509.127a1353@gandalf.local.home>
From:   Qing Zhang <zhangqing@loongson.cn>
Message-ID: <4b422c01-d8a1-8543-9f06-25a955f3686d@loongson.cn>
Date:   Sat, 20 Aug 2022 09:35:23 +0800
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On 2022/8/20 上午1:25, Steven Rostedt wrote:
> On Fri, 19 Aug 2022 16:13:55 +0800
> Qing Zhang <zhangqing@loongson.cn> wrote:
> 
>> +#define MCOUNT_STACK_SIZE	(2 * SZREG)
>> +#define MCOUNT_S0_OFFSET	(0)
>> +#define MCOUNT_RA_OFFSET	(SZREG)
>> +
>> +	.macro MCOUNT_SAVE_REGS
>> +	PTR_ADDI sp, sp, -MCOUNT_STACK_SIZE
>> +	PTR_S	s0, sp, MCOUNT_S0_OFFSET
>> +	PTR_S	ra, sp, MCOUNT_RA_OFFSET
>> +	move	s0, a0
>> +	.endm
>> +
>> +	.macro MCOUNT_RESTORE_REGS
>> +	move	a0, s0
>> +	PTR_L	ra, sp, MCOUNT_RA_OFFSET
>> +	PTR_L	s0, sp, MCOUNT_S0_OFFSET
>> +	PTR_ADDI sp, sp, MCOUNT_STACK_SIZE
>> +	.endm
>> +
>> +
>> +SYM_FUNC_START(_mcount)
>> +	la	t1, ftrace_stub
>> +	la	t2, ftrace_trace_function	/* Prepare t2 for (1) */
>> +	PTR_L	t2, t2, 0
>> +	beq	t1, t2, fgraph_trace
>> +
>> +	MCOUNT_SAVE_REGS
>> +
>> +	move	a0, ra				/* arg0: self return address */
>> +	move	a1, s0				/* arg1: parent's return address */
>> +	jirl	ra, t2, 0			/* (1) call *ftrace_trace_function */
>> +
>> +	MCOUNT_RESTORE_REGS
> 
> You know, if you can implement CONFIG_FTRACE_WITH_ARGS, where the default
> function callback gets a ftrace_regs pointer (that only holds what is
> needed for the arguments of the function as well as the stack pointer),
> then you could also implement function graph on top of that, and remove the
> need for the below "fgraph_trace" trampoline.
> 
> I'd really would like all architectures to go that way. Also, the
> CONFIG_FTRACE_WITH_ARGS is all you need for live kernel patching.
> 
Hi, Steve
I will add the implementation of CONFIG_FTRACE_WITH_ARGS in v2.

Thanks,
- Qing
> -- Steve
> 
> 
>> +
>> +fgraph_trace:
>> +#ifdef	CONFIG_FUNCTION_GRAPH_TRACER
>> +	la	t1, ftrace_stub
>> +	la	t3, ftrace_graph_return
>> +	PTR_L	t3, t3, 0
>> +	bne	t1, t3, ftrace_graph_caller
>> +	la	t1, ftrace_graph_entry_stub
>> +	la	t3, ftrace_graph_entry
>> +	PTR_L	t3, t3, 0
>> +	bne	t1, t3, ftrace_graph_caller
>> +#endif
>> +
>> +	.globl ftrace_stub
>> +ftrace_stub:
>> +	jirl	zero, ra, 0
>> +SYM_FUNC_END(_mcount)
>> +EXPORT_SYMBOL(_mcount)
>> +
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +SYM_FUNC_START(ftrace_graph_caller)
>> +	MCOUNT_SAVE_REGS
>> +
>> +	PTR_ADDI	a0, ra, -4			/* arg0: Callsite self return addr */
>> +	PTR_ADDI	a1, sp, MCOUNT_STACK_SIZE	/* arg1: Callsite sp */
>> +	move	a2, s0					/* arg2: Callsite parent ra */
>> +	bl	prepare_ftrace_return
>> +
>> +	MCOUNT_RESTORE_REGS
>> +	jirl	zero, ra, 0
>> +SYM_FUNC_END(ftrace_graph_caller)