Hi! On Wed, Nov 27, 2019 at 04:15:15PM +0100, Christophe Leroy wrote: > Le 27/11/2019 à 15:59, Segher Boessenkool a écrit : > >On Wed, Nov 27, 2019 at 02:50:30PM +0100, Christophe Leroy wrote: > >>So what do we do ? We just drop the "r2" clobber ? > > > >You have to make sure your asm code works for all ABIs. This is quite > >involved if you do a call to an external function. The compiler does > >*not* see this call, so you will have to make sure that all that the > >compiler and linker do will work, or prevent some of those things (say, > >inlining of the function containing the call). > > But the whole purpose of the patch is to inline the call to __do_irq() > in order to avoid the trampoline function.
Yes, so you call __do_irq. You have to make sure that what you tell the compiler -- and what you *don't tell the compiler -- works with what the ABIs require, and what the called function expects and provides. > >That does not fix everything. The called function requires a specific > >value in r2 on entry. > > Euh ... but there is nothing like that when using existing > call_do_irq(). > How does GCC know that call_do_irq() has same TOC as __do_irq() ? The existing call_do_irq isn't C code. It doesn't do anything with r2, as far as I can see; __do_irq just gets whatever the caller of call_do_irq has. So I guess all the callers of call_do_irq have the correct r2 value always already? In that case everything Just Works. > >So all this needs verification. Hopefully you can get away with just > >not clobbering r2 (and not adding a nop after the bl), sure. But this > >needs to be checked. > > > >Changing control flow inside inline assembler always is problematic. > >Another problem in this case (on all ABIs) is that the compiler does > >not see you call __do_irq. Again, you can probably get away with that > >too, but :-) > > Anyway it sees I reference it, as it is in input arguments. Isn't it > enough ? It is enough for some things, sure. But not all. Segher
