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SubjectRe: [PATCH 10/15] static_call: Add basic static call infrastructure
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. Furthermore,
how about we put the trampolines in .static_call.text instead of relying
on prefixes?

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.

- we can change static_call_key::site_mods into a single next pointer,
just like jump_label's static_key.

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.


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