Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop()

[Amit Kachhap]

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@xxxxxxx>
> > > Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > Cc: Torsten Duwe <duwe@xxxxxxx>
> > > ---
> > >    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.