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 eduma...@google.com> >>> Signed-off-by: Paolo Abeni <pab...@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 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.
> This: > https://lore.kernel.org/lkml/cover.1543200841.git.jpoim...@redhat.com/T/#ma30f6b2aa655c99e93cfb267fef75b8fe9fca29b > > is possibly related to what you are planning. Yes! That looks like exactly the sort of machinery I need, thanks. My idea is to build on that something that counts stats of where each indirect call goes, then every now and then patches in a new list of possible targets to an INDIRECT_CALL_*()-style jump tree, based on the N callees that were seen the most often. (Such a solution could even cope with callees in modules: the jump table doesn't need a reference on the module because it only calls the function (from the module) if the function pointer is pointing at it, in which case (hopefully) whoever supplied the function pointer has a reference to the module.) But I'm not sure I have the mad hacker skillz to make it work. -Ed
