On Thu, 2018-01-11 at 10:47 +0100, Borislav Petkov wrote:
> On Thu, Jan 11, 2018 at 10:32:31AM +0100, Peter Zijlstra wrote:
> >
> > can't you do lovely things like:
> >
> > volatile asm ("call __fill_rsb_thunk_%1" : : "r" (dummy))
> >
> > which would still let gcc select the register ?I've had to do that for the __x86_indirect_thunk_\reg thunks and provide all variants thereof, and export them to modules. I'd much rather have this one inline. If I couldn't do that (which I can, and have), then my next choice would probably have been to just hard-code it to use %eax/%r12 and emit a simple call to that from the call sites. > Calling a function from asm is nasty because you need to pay attention > to clobbered registers as gcc doesn't see the function. > > What one can do, I *think*, is do a non-inlined wrapping function and do > all the alternative_call() fun inside. There you can do all the fun and > have callee-clobbered regs which you can use. That's OK; it's not really a C function and it only clobbers the *one* register which is easy enough to tell __asm__() about.
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