Re: [PATCH v6 5/7] arm64: ftrace: Add dynamic ftrace support

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

Thank you for you clarification, Steven.

-Takahiro AKASHI

On 03/14/2014 03:33 AM, Steven Rostedt wrote:
> On Thu, 2014-03-13 at 18:10 +0000, Will Deacon wrote:
>>
>>> +#else /* CONFIG_DYNAMIC_FTRACE */
>>> +/*
>>> + * _mcount() is used to build the kernel with -pg option, but all the branch
>>> + * instructions to _mcount() are replaced to NOP initially at kernel start up,
>>> + * and later on, NOP to branch to ftrace_caller() when enabled or branch to
>>> + * NOP when disabled per-function base.
>>> + */
>>> +ENTRY(_mcount)
>>> +	ret
>>> +ENDPROC(_mcount)
>>
>> Judging by your comment then, this should never be called. Is that right? If
>> so, we could add a BUG-equivalent so we know if we missed an mcount during
>> patching.
>
> Actually, it can be called before the change to nops are done in early
> boot. This is done very early, but everything before ftrace_init() in
> init/main.c can still call _mcount.
>
>
>>> +	/*
>>> +	 * Note:
>>> +	 * Due to modules and __init, code can disappear and change,
>>> +	 * we need to protect against faulting as well as code changing.
>>> +	 * We do this by aarch64_insn_*() which use the probe_kernel_*().
>>> +	 *
>>> +	 * No lock is held here because all the modifications are run
>>> +	 * through stop_machine().
>>> +	 */
>>> +	if (validate) {
>>> +		if (aarch64_insn_read((void *)pc, &replaced))
>>> +			return -EFAULT;
>>> +
>>> +		if (replaced != old)
>>> +			return -EINVAL;
>>> +	}
>>> +	if (aarch64_insn_patch_text_nosync((void *)pc, new))
>>> +		return -EPERM;
>>
>> I think you're better off propagating the errors here, rather than
>> overriding them with EFAULT/EINVAL/EPERM.
>
> The ftrace generic code expects to see these specific errors. Look at
> ftrace_bug() in kernel/trace/ftrace.c.
>
>>
>>> +
>>> +	return 0;
>>> +}
>>> +
>>> +/*
>>> + * Replace tracer function in ftrace_caller()
>>> + */
>>> +int ftrace_update_ftrace_func(ftrace_func_t func)
>>> +{
>>> +	unsigned long pc;
>>> +	unsigned int new;
>>> +
>>> +	pc = (unsigned long)&ftrace_call;
>>> +	new = aarch64_insn_gen_branch_imm(pc, (unsigned long)func, true);
>>> +
>>> +	return ftrace_modify_code(pc, 0, new, false);
>>> +}
>>> +
>>> +/*
>>> + * Turn on the call to ftrace_caller() in instrumented function
>>> + */
>>> +int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
>>> +{
>>> +	unsigned long pc = rec->ip;
>>> +	unsigned int old, new;
>>> +
>>> +	old = aarch64_insn_gen_nop();
>>> +	new = aarch64_insn_gen_branch_imm(pc, addr, true);
>>> +
>>> +	return ftrace_modify_code(pc, old, new, true);
>>> +}
>>> +
>>> +/*
>>> + * Turn off the call to ftrace_caller() in instrumented function
>>> + */
>>> +int ftrace_make_nop(struct module *mod,
>>> +		    struct dyn_ftrace *rec, unsigned long addr)
>>> +{
>>> +	unsigned long pc = rec->ip;
>>> +	unsigned int old, new;
>>> +
>>> +	old = aarch64_insn_gen_branch_imm(pc, addr, true);
>>> +	new = aarch64_insn_gen_nop();
>>> +
>>> +	return ftrace_modify_code(pc, old, new, true);
>>> +}
>>> +
>>> +int __init ftrace_dyn_arch_init(void *data)
>>> +{
>>> +	*(unsigned long *)data = 0;
>>> +
>>> +	return 0;
>>> +}
>>> +#endif /* CONFIG_DYNAMIC_FTRACE */
>>> +
>>>   #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>>>   /*
>>>    * function_graph tracer expects ftrace_return_to_handler() to be called
>>> @@ -61,4 +144,34 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
>>>   		return;
>>>   	}
>>>   }
>>> +
>>> +#ifdef CONFIG_DYNAMIC_FTRACE
>>> +/*
>>> + * Turn on/off the call to ftrace_graph_caller() in ftrace_caller()
>>> + * depending on @enable.
>>> + */
>>> +static int ftrace_modify_graph_caller(bool enable)
>>> +{
>>> +	unsigned long pc = (unsigned long)&ftrace_graph_call;
>>> +	unsigned int branch, nop, old, new;
>>> +
>>> +	branch = aarch64_insn_gen_branch_imm(pc,
>>> +			(unsigned long)ftrace_graph_caller, false);
>>> +	nop = aarch64_insn_gen_nop();
>>> +	old = enable ? nop : branch;
>>> +	new = enable ? branch : nop;
>>> +
>>> +	return ftrace_modify_code(pc, old, new, true);
>>
>> You could rewrite this as:
>>
>> 	if (enable)
>> 		return ftrace_modify_code(pc, nop, branch, true);
>> 	else
>> 		return ftrace_modify_code(pc, branch, nop, true);
>>
>> which I find easier to read.
>
> Heh, maybe that could be updated in other archs too. I'll have to think
> about that one.
>
> -- Steve
>
>