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.
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.
Reviewed-by: Nicholas Piggin <npig...@gmail.com>
