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@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.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 INDIRECT_CALL_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.

At least the commit message should explain the rationale for not
 doing things this way.

-Ed

PS: this has reminded me of my desire to try runtime creation of
 these kinds of branch tables with self-modifying code; is there
 any documentation on how to go about writing to kernel .text at
 runtime?  Last time I had a try at it I got very confused.