Re: [RFC WIP PATCH] powerpc/32: system call implement entry/exit logic in C

[Christophe Leroy]

Le 03/04/2020 Ã 09:33, Nicholas Piggin a ÃcritÂ:
> Christophe Leroy's on April 1, 2020 9:48 pm:
> > Le 31/03/2020 Ã 17:22, Christophe Leroy a ÃcritÂ:
> > > That's first try to port PPC64 syscall entry/exit logic in C to PPC32.
> > > I've do the minimum to get it work. I have not reworked calls
> > > to sys_fork() and friends for instance.
> > >
> > > For the time being, it seems to work more or less but:
> > > - ping reports EINVAL on recvfrom
> > > - strace shows NULL instead of strings in call like open() for instance.
> >
> > For the two above problems, that's because system_call_exception()
> > doesn't set orig_gpr3 whereas DoSycall() does in entry_32.S . Is that
> > only done on PPC32 ?
> >
> > With the following line at the begining of system_call_exception(), it
> > works perfectly:
> >
> > 	regs->orig_gpr3 = r3;
>
> Oh great, nice work. We should be able to make some simple helpers or
> move some things a bit to reduce the amount of ifdefs in the C code.
> It doesn't look too bad though.
>
> > I will now focus on performance to see if we can do something about it.
>
> What's the performance difference between current asm code just with
> always saving NVGPRS vs C?

Done new measurement and sent a series. lower values now because the 
time accounting was done twice as it was still in the ASM part.

Before the series, 311 cycles for a null_syscall
If adding SAVE_NVGPRS to the entry macro, 335 cycles

First patch: 353 cycles ie +13,5%

After a few changes, including conditional saving of non volatile 
registers, I get 325 cycles that is only +4,5%. I thing that's acceptable.

Do you see a problem with still saving non volatile registers only when 
required ?

Christophe