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Date: Wed, 06 Jul 2016 12:58:11 +0800
From: xinhui <xinhui.pan@linux.vnet.ibm.com>
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Subject: Re: [PATCH v2 2/4] powerpc/spinlock: support vcpu preempted check
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To: Wanpeng Li <kernellwp@gmail.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, linuxppc-dev@lists.ozlabs.org, virtualization@lists.linux-foundation.org, linux-s390 <linux-s390@vger.kernel.org>, Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, mpe@ellerman.id.au, Paul Mackerras <paulus@samba.org>, benh@kernel.crashing.org, Paul McKenney <paulmck@linux.vnet.ibm.com>, Waiman Long <waiman.long@hpe.com>, will.deacon@arm.com, boqun.feng@gmail.com, Davidlohr Bueso <dave@stgolabs.net>, schwidefsky@de.ibm.com
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Hi, wanpeng

On 2016年07月05日 17:57, Wanpeng Li wrote:
> Hi Xinhui,
> 2016-06-28 22:43 GMT+08:00 Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>:
>> This is to fix some lock holder preemption issues. Some other locks
>> implementation do a spin loop before acquiring the lock itself. Currently
>> kernel has an interface of bool vcpu_is_preempted(int cpu). It take the cpu
>> as parameter and return true if the cpu is preempted. Then kernel can break
>> the spin loops upon on the retval of vcpu_is_preempted.
>>
>> As kernel has used this interface, So lets support it.
>>
>> Only pSeries need supoort it. And the fact is powerNV are built into same
>> kernel image with pSeries. So we need return false if we are runnig as
>> powerNV. The another fact is that lppaca->yiled_count keeps zero on
>> powerNV. So we can just skip the machine type.
>
> Lock holder vCPU preemption can be detected by hardware pSeries or
> paravirt method?
>
There is one shard struct between kernel and powerVM/KVM. And we read the yield_count of this struct to detect if one vcpu is running or not.
SO it's easy for ppc to implement such interface. Note that yield_count is set by powerVM/KVM.
and only pSeries can run a guest for now. :)

I also review x86 related code, looks like we need add one hyer-call to get such vcpu preemption info?

thanks
xinui
> Regards,
> Wanpeng Li
>