On Fri, Nov 30, 2018 at 11:16:34PM +0100, Rasmus Villemoes wrote: > On 29/11/2018 20.22, Josh Poimboeuf wrote: > > On Thu, Nov 29, 2018 at 02:16:48PM -0500, Steven Rostedt wrote: > >>> and honestly, the way "static_call()" works now, can you guarantee > >>> that the call-site doesn't end up doing that, and calling the > >>> trampoline function for two different static calls from one indirect > >>> call? > >>> > >>> See what I'm talking about? Saying "callers are wrapped in macros" > >>> doesn't actually protect you from the compiler doing things like that. > >>> > >>> In contrast, if the call was wrapped in an inline asm, we'd *know* the > >>> compiler couldn't turn a "call wrapper(%rip)" into anything else. > >> > >> But then we need to implement all numbers of parameters. > > > > I actually have an old unfinished patch which (ab)used C macros to > > detect the number of parameters and then setup the asm constraints > > accordingly. At the time, the goal was to optimize the BUG code. > > > > I had wanted to avoid this kind of approach for static calls, because > > "ugh", but now it's starting to look much more appealing. > > > > Behold: > > > > diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h > > index aa6b2023d8f8..d63e9240da77 100644 > > --- a/arch/x86/include/asm/bug.h > > +++ b/arch/x86/include/asm/bug.h > > @@ -32,10 +32,59 @@ > > > > #ifdef CONFIG_DEBUG_BUGVERBOSE > > > > -#define _BUG_FLAGS(ins, flags) > > \ > > +#define __BUG_ARGS_0(ins, ...) \ > > +({\ > > + asm volatile("1:\t" ins "\n"); \ > > +}) > > +#define __BUG_ARGS_1(ins, ...) \ > > +({\ > > + asm volatile("1:\t" ins "\n" \ > > + : : "D" (ARG1(__VA_ARGS__))); \ > > +}) > > +#define __BUG_ARGS_2(ins, ...) \ > > +({\ > > + asm volatile("1:\t" ins "\n" \ > > + : : "D" (ARG1(__VA_ARGS__)), \ > > + "S" (ARG2(__VA_ARGS__))); \ > > +}) > > +#define __BUG_ARGS_3(ins, ...) \ > > +({\ > > + asm volatile("1:\t" ins "\n" \ > > + : : "D" (ARG1(__VA_ARGS__)), \ > > + "S" (ARG2(__VA_ARGS__)), \ > > + "d" (ARG3(__VA_ARGS__))); \ > > +}) > > wouldn't you need to tie all these to (unused) outputs as well as adding > the remaining caller-saved registers to the clobber list? Maybe not for > the WARN machinery(?), but at least for stuff that should look like a > normal call to gcc? Then there's %rax which is either a clobber or an > output, and if there's not to be a separate static_call_void(), one > would need to do some __builtin_choose_expr(__same_type(void, f(...)), ...).
Yes, this is a crappy unfinished patch. It should be ignored, and perhaps even mercilessly mocked :-) paravirt_types.h already does something similar today, and it's at least more correct than this. What I was trying to show was that you can use macros to count arguments, like this: _BUG_ARGS(ins, NUM_ARGS(__VA_ARGS__), __VA_ARGS__); which can make a macro look and act like a function call. Though as Steven pointed out, the concept falls apart after 6 arguments. -- Josh