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

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Date:   Wed, 05 Dec 2018 19:03:28 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Edward Cree <ecree@...arflare.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 1/4] indirect call wrappers: helpers to
 speed-up indirect calls of builtin

On Tue, 2018-12-04 at 18:12 +0000, Edward Cree wrote:
> On 04/12/18 17:44, Paolo Abeni wrote:
> > On Tue, 2018-12-04 at 17:13 +0000, Edward Cree wrote:
> > > On 03/12/18 11:40, 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 macros defined here implement the boilerplate for the above schema
> > > > and will be used by the next patches.
> > > > 
> > > > rfc -> v1:
> > > >  - use branch prediction hint, as suggested by Eric
> > > > 
> > > > Suggested-by: Eric Dumazet <Eric Dumazet edumazet@...gle.com>
> > > > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > > > ---
> > > I'm not sure I see the reason why this is done with numbers and
> > >  'name ## NR', adding extra distance between the callsite and the
> > >  list of callees.  In particular it means that each callable needs
> > >  to specify its index.
> > > Wouldn't it be simpler just to have
> > >     #define 1(f, f1, ...) \
> > >         (likely(f == f1) ? f1(__VA_ARGS__) : f(__VA_ARGS__))
> > >     #define INDIRECT_CALL_2(f, f2, f1, ...) \
> > >         (likely(f == f2) ? f2(__VA_ARGS__) : INDIRECT_CALL_1(f, f1, __VA_ARGS__))
> > > etc.?  Removing the need for INDIRECT_CALLABLE_DECLARE_* entirely.
> > Thank you for the review!
> > 
> > As some of the builtin symbols are static, we would still need some
> > macro wrappers to properly specify the scope when retpoline is enabled.
> Ah I see, it hadn't occurred to me that static callees might not be
>  available at the callsite.  Makes sense now.  In that case, have my
>  Acked-By for this patch, if you want it.

I gave a shot to your idea, and after all I think is cleaner. So I'll
send v2 with that change.

Thanks,

Paolo