Re: Use of SPRG0-3 on PowerPC & SMP issues

[ws%tools.de@localhost (Wolfgang Solfrank)]

Hi,

While I can't argue that the trap handlers could stand quite some cleanup
(and not only for SMP purposes I'd say), I've got some comments on your
analysis:

> The first is that the relative lack of use of the SPRG0-3 registers
> by NetBSD.  Only SPRG1 is currently in use in trap_subr.S to store
> away the SP (reg 1).

I suggest you look again.  SPRG2 is used to save XER temporarily away,
and SPRG2 & 3 are used during return from the traps to hold r2 & r3 contents.

> 3) (this just because it bugs me).  curpm is changed to paddr_t instead of
>     struct pmap *, since it's the physical address of the current pmap and 
> then
>     a bunch of casts can disappear.

Hmm, not sure on this one.  (BTW, you probably mean pcb_pmreal and as
a result curpm, as there currently is no cast involved when accessing
curpm.)  Conceptually, with NetBSD/powerpc, there is no difference
between physical and virtual addresses.  And there _are_ accesses
to the pmap via the pcb_pmreal pointer, which would require a cast
if you changed this.

> This should actually result in a slightly faster system since a number of
> global references will become register indexed.

Don't count on that.  The tempsave (et. al.) area is deliberately placed
into locore, so it can be accessed without loading the high part of the
address.  So actually accessing this area needs quite a bit more work
than the current code:  you have to get yourself another free register
for loading the cpu_info pointer out of SPRG3, then probably another one
to load the tempsave pointer out of cpu_info.  Getting the intstk out
of cpu_info will save you an instruction though (albeit only on
interrupts, not on other traps).

In total, I'd be very much surprised if the system would get faster
by this (assuming no other optimizations).

Ciao,
Wolfgang
-- 
ws%TooLs.DE@localhost     Wolfgang Solfrank, TooLs GmbH         +49-228-985800