[RFC PATCH 2/2] powerpc: nop trap instruction after WARN_ONCE fires

[Nicholas Piggin]

On Sat Sep 24, 2022 at 2:47 AM AEST, Christophe Leroy wrote:
>
>
> Le 23/09/2022 à 17:41, Nicholas Piggin a écrit :
> > WARN_ONCE and similar are often used in frequently executed code, and
> > should not crash the system. The program check interrupt caused by
> > WARN_ON_ONCE can be a significant overhead even when nothing is being
> > printed. This can cause performance to become unacceptable, having the
> > same effective impact to the user as a BUG_ON().
> > 
> > Avoid this overhead by patching the trap with a nop instruction after a
> > "once" trap fires. Conditional warnings that return a result must have
> > equivalent compare and branch instructions after the trap, so when it is
> > nopped the statement will behave the same way. It's possible the asm
> > goto should be removed entirely and this comparison just done in C now.
>
> You mean, just like PPC32 ? (Since db87a7199229 ("powerpc/bug: Remove 
> specific powerpc BUG_ON() and WARN_ON() on PPC32"))
>
> But I'm having hard time with your change.
>
> You change only WARN_ON()
> But WARN_ON_ONCE() calls __WARN_FLAGS()
> And WARN_ONCE() calls WARN() via DO_ONCE_LITE_IF()
>
> So I don't see any ..._ONCE something going with WARN_ON().
>
> Am I missing something ?

Hmm, no I must have missed something. I guess it is the EMIT_WARN_ENTRY
in asm which is the main problem I've seen. Although we could remove the
DO_ONCE_LITE_IF code generation from our WARN_ON_ONCE as well if we did
this patching.

Thanks,
Nick