Excerpts from Christophe Leroy's message of February 10, 2021 2:13 am: > > > Le 09/02/2021 à 02:55, Nicholas Piggin a écrit : >> Excerpts from Christophe Leroy's message of February 9, 2021 1:10 am: >>> When r3 is not modified, reload it from regs->orig_r3 to free >>> volatile registers. This avoids a stack frame for the likely part >>> of system_call_exception() >> >> This doesn't on my 64s build, but it does reduce one non volatile >> register save/restore. With quite a bit more register pressure >> reduction 64s can avoid the stack frame as well. > > The stack frame is not due to the registers because on PPC64 you have the > redzone that you don't > have on PPC32. > > As far as I can see, this is due to a call to .arch_local_irq_restore(). > > On ppc32 arch_local_irq_restore() is just a write to MSR.
Oh you're right there. We can actually inline fast paths of that I have a patch somewhere, but not sure if it's worthwhile. >> It's a cool trick but quite code and compiler specific so I don't know >> how worthwhile it is to keep considering we're calling out into random >> kernel C code after this. >> >> Maybe just keep it PPC32 specific for the moment, will have to do more >> tuning for 64 and we have other stuff to do there first. >> >> If you are happy to make it 32-bit only then > > I think we can leave without this, that's only one or two cycles won. Okay for this round let's drop it for now. Thanks, Nick
