Re: [PATCH v5 17/22] powerpc/syscall: Do not check unsupported scv vector on PPC32

[Christophe Leroy]

Le 09/02/2021 à 03:00, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of February 9, 2021 1:10 am:
> > Only PPC64 has scv. No need to check the 0x7ff0 trap on PPC32.
> > For that, add a helper trap_is_unsupported_scv() similar to
> > trap_is_scv().
> >
> > And ignore the scv parameter in syscall_exit_prepare (Save 14 cycles
> > 346 => 332 cycles)
> >
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> > v5: Added a helper trap_is_unsupported_scv()
> > ---
> >   arch/powerpc/include/asm/ptrace.h | 5 +++++
> >   arch/powerpc/kernel/entry_32.S    | 1 -
> >   arch/powerpc/kernel/interrupt.c   | 7 +++++--
> >   3 files changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
> > index 58f9dc060a7b..2c842b11a924 100644
> > --- a/arch/powerpc/include/asm/ptrace.h
> > +++ b/arch/powerpc/include/asm/ptrace.h
> > @@ -229,6 +229,11 @@ static inline bool trap_is_scv(struct pt_regs *regs)
> >   	return (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && TRAP(regs) == 0x3000);
> >   }
> >   
> > +static inline bool trap_is_unsupported_scv(struct pt_regs *regs)
> > +{
> > +	return (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && TRAP(regs) == 0x7ff0);
> > +}
>
> This change is good.
>
> > +
> >   static inline bool trap_is_syscall(struct pt_regs *regs)
> >   {
> >   	return (trap_is_scv(regs) || TRAP(regs) == 0xc00);
> > diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> > index cffe58e63356..7c824e8928d0 100644
> > --- a/arch/powerpc/kernel/entry_32.S
> > +++ b/arch/powerpc/kernel/entry_32.S
> > @@ -344,7 +344,6 @@ transfer_to_syscall:
> >   
> >   ret_from_syscall:
> >   	addi    r4,r1,STACK_FRAME_OVERHEAD
> > -	li	r5,0
> >   	bl	syscall_exit_prepare
>
> For this one, I think it would be nice to do the "right" thing and make
> the function prototypes different on !64S. They could then declare a
> local const bool scv = 0.
>
> We could have syscall_exit_prepare and syscall_exit_prepare_maybe_scv
> or something like that, 64s can use the latter one and the former can be
> a wrapper that passes constant 0 for scv. Then we don't have different
> prototypes for the same function, but you just have to make the 32-bit
> version static inline and the 64-bit version exported to asm.

You can't call a static inline function from ASM, I don't understand you.

What is wrong for you really here ? Is that the fact we leave scv random, or is that the below 
IS_ENABLED() ?

I don't mind keeping the 'li r5,0' before calling the function if you find it cleaner, the real 
performance gain is with setting scv to 0 below for PPC32 (and maybe it should be set to zero for 
book3e/64 too ?).

Other solution would be to do replace (!scv) by (!scv || !IS_ENABLED(CONFIG_PPC_BOOK3S_64)) in the 
two places it is used in syscall_exit_prepare().

Any preference ?

Thanks
Christophe

> > @@ -224,6 +224,9 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
> >   	unsigned long ti_flags;
> >   	unsigned long ret = 0;
> >   
> > +	if (IS_ENABLED(CONFIG_PPC32))
> > +		scv = 0;
> > +
> >   	CT_WARN_ON(ct_state() == CONTEXT_USER);
> >   
> >   #ifdef CONFIG_PPC64
> > --
> > 2.25.0