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

From: Michael Ellerman <mpe@ellerman.id.au>
To: linuxppc-dev@lists.ozlabs.org
Cc: Nicholas Piggin <npiggin@gmail.com>,
	Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@redhat.com>,
	Waiman Long <longman@redhat.com>,
	Anton Blanchard <anton@ozlabs.org>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR
Date: Thu, 09 Jul 2020 20:53:16 +1000	[thread overview]
Message-ID: <874kqhvu1v.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <20200706043540.1563616-6-npiggin@gmail.com>

Nicholas Piggin <npiggin@gmail.com> writes:

> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  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?

> 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?

> 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
