Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq()

Le 27/11/2019 à 15:59, Segher Boessenkool a écrit :

On Wed, Nov 27, 2019 at 02:50:30PM +0100, Christophe Leroy wrote:

quoted

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.

quoted

Otherwise, to be on the safe side we can just save r2 in a local var
before the bl and restore it after. I guess it won't collapse CPU time
on a performant PPC64.

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() ?

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 ?

Christophe