On Wed, Oct 02, 2019 at 03:54:17PM +0200, Peter Zijlstra wrote: > On Fri, Jun 07, 2019 at 10:28:51AM +0200, Peter Zijlstra wrote: > > On Thu, Jun 06, 2019 at 10:44:23PM +0000, Nadav Amit wrote: > > > > + * Usage example: > > > > + * > > > > + * # Start with the following functions (with identical prototypes): > > > > + * int func_a(int arg1, int arg2); > > > > + * int func_b(int arg1, int arg2); > > > > + * > > > > + * # Define a 'my_key' reference, associated with func_a() by default > > > > + * DEFINE_STATIC_CALL(my_key, func_a); > > > > + * > > > > + * # Call func_a() > > > > + * static_call(my_key, arg1, arg2); > > > > + * > > > > + * # Update 'my_key' to point to func_b() > > > > + * static_call_update(my_key, func_b); > > > > + * > > > > + * # Call func_b() > > > > + * static_call(my_key, arg1, arg2); > > > > > > I think that this calling interface is not very intuitive. > > > > Yeah, it is somewhat unfortunate.. > > > > > I understand that > > > the macros/objtool cannot allow the calling interface to be completely > > > transparent (as compiler plugin could). But, can the macros be used to > > > paste the key with the “static_call”? I think that having something like: > > > > > > static_call__func(arg1, arg2) > > > > > > Is more readable than > > > > > > static_call(func, arg1, arg2) > > > > Doesn't really make it much better for me; I think I'd prefer to switch > > to the GCC plugin scheme over this. ISTR there being some propotypes > > there, but I couldn't quickly locate them. > > How about something like: > > static_call(key)(arg1, arg2); > > which is very close to the regular indirect call syntax.
Looks ok to me. > Furthermore, how about we put the trampolines in .static_call.text > instead of relying on prefixes? Yeah, that would probably be better. > Also, I think I can shrink static_call_key by half: > > - we can do away with static_call_key::tramp; there are only two usage > sites: > > o __static_call_update, the static_call() macro can provide the > address of STATIC_CALL_TRAMP(key) directly > > o static_call_add_module(), has two cases: > > * the trampoline is from outside the module; in this case > it will already have been updated by a previous call to > __static_call_update. > * the trampoline is from inside the module; in this case > it will have the default value and it doesn't need an > update. > > so in no case does static_call_add_module() need to modify a > trampoline. Sounds plausible. > - we can change static_call_key::site_mods into a single next pointer, > just like jump_label's static_key. Yep. > But so far all the schemes I've come up with require 'key' to be a name, > it cannot be an actual 'struct static_call_key *' value. And therefore > usage from within structures isn't allowed. Is that something we need? At least we were able to work around this limitation with tracepoints' usage of static calls. But I could see how it could be useful. One way to solve that would be a completely different implementation: have a global trampoline which detects the call site of the caller, associates it with the given key, schedules some work to patch the call site later, and then jumps to key->func. So the first call would trigger the patching. Then we might not even need objtool :-) But it might be tricky to pull off. -- Josh
