Date:   Tue, 5 Nov 2019 12:17:26 +0530
From:   Amit Kachhap <amit.kachhap@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, catalin.marinas@....com,
        deller@....de, duwe@...e.de, James.Bottomley@...senPartnership.com,
        james.morse@....com, jeyu@...nel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, linux-parisc@...r.kernel.org,
        mingo@...hat.com, peterz@...radead.org, svens@...ckframe.org,
        takahiro.akashi@...aro.org, will@...nel.org
Subject: Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop()

Hi Mark,

On 11/4/19 7:06 PM, Mark Rutland wrote:
> On Sat, Nov 02, 2019 at 05:49:00PM +0530, Amit Daniel Kachhap wrote:
>> Hi Mark,
> 
> Hi Amit,
> 
> Thanks for the review!
>   
>> On 10/29/19 10:28 PM, Mark Rutland wrote:
>>> Architectures may need to perform special initialization of ftrace
>>> callsites, and today they do so by special-casing ftrace_make_nop() when
>>> the expected branch address is MCOUNT_ADDR. In some cases (e.g. for
>>> patchable-function-entry), we don't have an mcount-like symbol and don't
>> s/an mcount/a mcount.
>>> want a synthetic MCOUNT_ADDR, but we may need to perform some
>>> initialization of callsites.
>>>
>>> To make it possible to separate initialization from runtime
>>> modification, and to handle cases without an mcount-like symbol, this
>> Same as above.
> 
> Using 'an' in both of these cases is correct, even though 'mcount'
> starts with a consonant, since it's pronounced as-if it were 'emcount'.
> I will leave this as-is.
Ok sure. It makes sense.
> 
>>> patch adds an optional ftrace_init_nop() function that architectures can
>>> implement, which does not pass a branch address.
>>>
>>> Where an architecture does not provide ftrace_init_nop(), we will fall
>>> back to the existing behaviour of calling ftrace_make_nop() with
>>> MCOUNT_ADDR.
>>>
>>> At the same time, ftrace_code_disable() is renamed to
>>> ftrace_nop_initialize() to make it clearer that it is intended to
>>> intialize a callsite into a disabled state, and is not for disabling a
>>> callsite that has been runtime enabled. The kerneldoc description of rec
>>> arguments is updated to cover non-mcount callsites.
>>>
>>> Signed-off-by: Mark Rutland <mark.rutland@....com>
>>> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: Steven Rostedt <rostedt@...dmis.org>
>>> Cc: Torsten Duwe <duwe@...e.de>
>>> ---
>>>    include/linux/ftrace.h | 35 ++++++++++++++++++++++++++++++++---
>>>    kernel/trace/ftrace.c  |  6 +++---
>>>    2 files changed, 35 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
>>> index 8a8cb3c401b2..9867d90d635e 100644
>>> --- a/include/linux/ftrace.h
>>> +++ b/include/linux/ftrace.h
>>> @@ -499,7 +499,7 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
>>>    /**
>>>     * ftrace_make_nop - convert code into nop
>>>     * @mod: module structure if called by module load initialization
>>> - * @rec: the mcount call site record
>>> + * @rec: the call site record (e.g. mcount/fentry)
>>>     * @addr: the address that the call site should be calling
>>>     *
>>>     * This is a very sensitive operation and great care needs
>>> @@ -520,9 +520,38 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
>>>    extern int ftrace_make_nop(struct module *mod,
>>>    			   struct dyn_ftrace *rec, unsigned long addr);
>>> +
>>> +/**
>>> + * ftrace_init_nop - initialize a nop call site
>>> + * @mod: module structure if called by module load initialization
>>> + * @rec: the call site record (e.g. mcount/fentry)
>>> + *
>>> + * This is a very sensitive operation and great care needs
>>> + * to be taken by the arch.  The operation should carefully
>>> + * read the location, check to see if what is read is indeed
>>> + * what we expect it to be, and then on success of the compare,
>>> + * it should write to the location.
>>> + *
>>> + * The code segment at @rec->ip should contain the contents created by
>>> + * the compiler
>> Nit: Will it be better to write it as "@rec->ip should store the adjusted
>> ftrace entry address of the call site" or something like that.
> 
> This was the specific wording requested by Steve, and it's trying to
> describe the instructions at rec->ip, rather than the value of rec->ip,
> so I think it's better to leave this as-is.
ok Its fine this way too. Actually from the comment, I could not 
understand which one of the compiler contents this points to as in this 
case there are 2 nops.
> 
>>> + *
>>> + * Return must be:
>>> + *  0 on success
>>> + *  -EFAULT on error reading the location
>>> + *  -EINVAL on a failed compare of the contents
>>> + *  -EPERM  on error writing to the location
>>> + * Any other value will be considered a failure.
>>> + */
>>> +#ifndef ftrace_init_nop
>>> +static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
>>> +{
>>> +	return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
>>> +}
>>> +#endif
>>> +
>> Now that ftrace_init_nop is also an arch implemented function so it may be
>> added in Documentation/trace/ftrace-design.rst along with ftrace_make_nop.
>> In general also, adding some description about patchable-function-entry
>> in kernel Documentation will be useful.
> 
> I agree that would be a good thing, but as Steve suggests I will leave
> this for subsequent rework, as I think that also implies more
> rework/renaming in the core code (e.g. to more cleanly separate the
> notion of a callsite from mcount specifically).
ok.

Thanks,
Amit
> 
> Steve, if you'd like help with that (or review), I'd be happy to help.
> 
> Thanks,
> Mark.
> 

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