netdev - Re: [RFC PATCH 1/4] indirect call wrappers: helpers to speed-up indirect calls of builtin

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Date:   Fri, 30 Nov 2018 09:29:57 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC PATCH 1/4] indirect call wrappers: helpers to speed-up
 indirect calls of builtin

Hi,

On Thu, 2018-11-29 at 15:25 -0800, Eric Dumazet wrote:
> 
> On 11/29/2018 03:00 PM, Paolo Abeni wrote:
> > This header define a bunch of helpers that allow avoiding the
> > retpoline overhead when calling builtin functions via function pointers.
> > It boils down to explicitly comparing the function pointers to
> > known builtin functions and eventually invoke directly the latter.
> > 
> > The macro defined here implement the boilerplate for the above schema
> > and will be used by the next patches.
> > 
> > Suggested-by: Eric Dumazet <Eric Dumazet <edumazet@...gle.com>

Oops... typo here. For some reasons checkpatch did not catch it.

> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> >  include/linux/indirect_call_wrapper.h | 77 +++++++++++++++++++++++++++
> >  1 file changed, 77 insertions(+)
> >  create mode 100644 include/linux/indirect_call_wrapper.h
> > 
> > diff --git a/include/linux/indirect_call_wrapper.h b/include/linux/indirect_call_wrapper.h
> > new file mode 100644
> > index 000000000000..57e82b4a166d
> > --- /dev/null
> > +++ b/include/linux/indirect_call_wrapper.h
> > @@ -0,0 +1,77 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _LINUX_INDIRECT_CALL_WRAPPER_H
> > +#define _LINUX_INDIRECT_CALL_WRAPPER_H
> > +
> > +#ifdef CONFIG_RETPOLINE
> > +
> > +/*
> > + * INDIRECT_CALL_$NR - wrapper for indirect calls with $NR known builtin
> > + *  @f: function pointer
> > + *  @name: base name for builtin functions, see INDIRECT_CALLABLE_DECLARE_$NR
> > + *  @__VA_ARGS__: arguments for @f
> > + *
> > + * Avoid retpoline overhead for known builtin, checking @f vs each of them and
> > + * eventually invoking directly the builtin function. Fallback to the indirect
> > + * call
> > + */
> > +#define INDIRECT_CALL_1(f, name, ...)					\
> > +	({								\
> > +		f == name ## 1 ? name ## 1(__VA_ARGS__) :		\
> 
>               likely(f == name ## 1) ? ...

Thank you for the feedback!

I thought about the above, and than I avoided it, because I was not
100% it would fit cases (if any) where we have 2 or more built-in
equally likely.

I guess we can address such cases if and when they will pop-up. I'll do
some more benchmarks with the branch prediction hints, and then if
there are no surprises, I'll add them in v1.

BTW I would like to give the correct attribution here. Does 'Suggested-
by' fit? should I list some other guy @google?

Thanks,

Paols