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

From: Waiman Long <longman@redhat.com>
To: Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	linuxppc-dev@lists.ozlabs.org
Cc: Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@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, 9 Jul 2020 12:06:13 -0400	[thread overview]
Message-ID: <8265d782-4e50-a9b2-a908-0cb588ffa09c@redhat.com> (raw)
In-Reply-To: <874kqhvu1v.fsf@mpe.ellerman.id.au>

On 7/9/20 6:53 AM, Michael Ellerman wrote:
> 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?
>
I am OK with adding the #ifdef around queued_spin_lock().

Acked-by: Waiman Long <longman@redhat.com>

>> 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