Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR

[Waiman Long]

On 7/9/20 6:53 AM, Michael Ellerman wrote:
> Nicholas Piggin <npiggin@xxxxxxxxx> writes:
>
> > Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> > ---
> >   arch/powerpc/include/asm/paravirt.h           | 28 ++++++++
> >   arch/powerpc/include/asm/qspinlock.h          | 66 +++++++++++++++++++
> >   arch/powerpc/include/asm/qspinlock_paravirt.h |  7 ++
> >   arch/powerpc/platforms/pseries/Kconfig        |  5 ++
> >   arch/powerpc/platforms/pseries/setup.c        |  6 +-
> >   include/asm-generic/qspinlock.h               |  2 +
> Another ack?
I am OK with adding the #ifdef around queued_spin_lock().

Acked-by: Waiman Long <longman@xxxxxxxxxx>

> > diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
> > index 7a8546660a63..f2d51f929cf5 100644
> > --- a/arch/powerpc/include/asm/paravirt.h
> > +++ b/arch/powerpc/include/asm/paravirt.h
> > @@ -45,6 +55,19 @@ static inline void yield_to_preempted(int cpu, u32 yield_count)
> >   {
> >   	___bad_yield_to_preempted(); /* This would be a bug */
> >   }
> > +
> > +extern void ___bad_yield_to_any(void);
> > +static inline void yield_to_any(void)
> > +{
> > +	___bad_yield_to_any(); /* This would be a bug */
> > +}
> Why do we do that rather than just not defining yield_to_any() at all
> and letting the build fail on that?
>
> There's a condition somewhere that we know will false at compile time
> and drop the call before linking?
>
> > diff --git a/arch/powerpc/include/asm/qspinlock_paravirt.h b/arch/powerpc/include/asm/qspinlock_paravirt.h
> > new file mode 100644
> > index 000000000000..750d1b5e0202
> > --- /dev/null
> > +++ b/arch/powerpc/include/asm/qspinlock_paravirt.h
> > @@ -0,0 +1,7 @@
> > +/* SPDX-License-Identifier: GPL-2.0-or-later */
> > +#ifndef __ASM_QSPINLOCK_PARAVIRT_H
> > +#define __ASM_QSPINLOCK_PARAVIRT_H
> _ASM_POWERPC_QSPINLOCK_PARAVIRT_H please.
>
> > +
> > +EXPORT_SYMBOL(__pv_queued_spin_unlock);
> Why's that in a header? Should that (eventually) go with the generic implementation?
The PV qspinlock implementation is not that generic at the moment. Even 
though native qspinlock is used by a number of archs, PV qspinlock is 
only currently used in x86. This is certainly an area that needs 
improvement.
> > diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
> > index 24c18362e5ea..756e727b383f 100644
> > --- a/arch/powerpc/platforms/pseries/Kconfig
> > +++ b/arch/powerpc/platforms/pseries/Kconfig
> > @@ -25,9 +25,14 @@ config PPC_PSERIES
> >   	select SWIOTLB
> >   	default y
> >   
> > +config PARAVIRT_SPINLOCKS
> > +	bool
> > +	default n
> default n is the default.
>
> > diff --git a/arch/powerpc/platforms/pseries/setup.c b/arch/powerpc/platforms/pseries/setup.c
> > index 2db8469e475f..747a203d9453 100644
> > --- a/arch/powerpc/platforms/pseries/setup.c
> > +++ b/arch/powerpc/platforms/pseries/setup.c
> > @@ -771,8 +771,12 @@ static void __init pSeries_setup_arch(void)
> >   	if (firmware_has_feature(FW_FEATURE_LPAR)) {
> >   		vpa_init(boot_cpuid);
> >   
> > -		if (lppaca_shared_proc(get_lppaca()))
> > +		if (lppaca_shared_proc(get_lppaca())) {
> >   			static_branch_enable(&shared_processor);
> > +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> > +			pv_spinlocks_init();
> > +#endif
> > +		}
> We could avoid the ifdef with this I think?
>
> diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
> index 434615f1d761..6ec72282888d 100644
> --- a/arch/powerpc/include/asm/spinlock.h
> +++ b/arch/powerpc/include/asm/spinlock.h
> @@ -10,5 +10,9 @@
>   #include <asm/simple_spinlock.h>
>   #endif
>
> +#ifndef CONFIG_PARAVIRT_SPINLOCKS
> +static inline void pv_spinlocks_init(void) { }
> +#endif
> +
>   #endif /* __KERNEL__ */
>   #endif /* __ASM_SPINLOCK_H */
>
>
> cheers
We don't really need to do a pv_spinlocks_init() if pv_kick() isn't 
supported.

Cheers,
Longman