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Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for
 SPLPAR
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        linuxppc-dev@lists.ozlabs.org, Will Deacon <will@kernel.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
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <845de183-56f5-2958-3159-faa131d46401@redhat.com>
Date:   Thu, 23 Jul 2020 14:32:36 -0400
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On 7/23/20 10:00 AM, Peter Zijlstra wrote:
> On Thu, Jul 09, 2020 at 12:06:13PM -0400, Waiman Long wrote:
>> We don't really need to do a pv_spinlocks_init() if pv_kick() isn't
>> supported.
> Waiman, if you cannot explain how not having kick is a sane thing, what
> are you saying here?
>
The current PPC paravirt spinlock code doesn't do any cpu kick. It does 
an equivalence of pv_wait by yielding the cpu to the lock holder only. 
The pv_spinlocks_init() is for setting up the hash table for doing 
pv_kick. If we don't need to do pv_kick, we don't need the hash table.

I am not saying that pv_kick is not needed for the PPC environment. I 
was just trying to adapt the pvqspinlock code to such an environment 
first. Further investigation on how to implement some kind of pv_kick 
will be something that we may want to do as a follow on.

BTW, do you have any comment on my v2 lock holder cpu info qspinlock 
patch? I will have to update the patch to fix the reported 0-day test 
problem, but I want to collect other feedback before sending out v3.

Cheers,
Longman