Re: [PATCH RFC v5 7/8] pvqspinlock, x86: Add qspinlock para-virtualization support

[Waiman Long <waiman.long@hp.com>]

On 02/27/2014 09:45 AM, Paolo Bonzini wrote:
> Il 27/02/2014 15:18, David Vrabel ha scritto:
>> On 27/02/14 13:11, Paolo Bonzini wrote:
>>> Il 27/02/2014 13:11, David Vrabel ha scritto:
>>>>>> This patch adds para-virtualization support to the queue spinlock 
>>>>>> code
>>>>>> by enabling the queue head to kick the lock holder CPU, if known,
>>>>>> in when the lock isn't released for a certain amount of time. It
>>>>>> also enables the mutual monitoring of the queue head CPU and the
>>>>>> following node CPU in the queue to make sure that their CPUs will
>>>>>> stay scheduled in.
>>>> I'm not really understanding how this is supposed to work.  There
>>>> appears to be an assumption that a guest can keep one of its VCPUs
>>>> running by repeatedly kicking it?  This is not possible under Xen 
>>>> and I
>>>> doubt it's possible under KVM or any other hypervisor.
>>>
>>> KVM allows any VCPU to wake up a currently halted VCPU of its choice,
>>> see Documentation/virtual/kvm/hypercalls.txt.
>>
>> But neither of the VCPUs being kicked here are halted -- they're either
>> running or runnable (descheduled by the hypervisor).
>
> /me actually looks at Waiman's code...
>
> Right, this is really different from pvticketlocks, where the *unlock* 
> primitive wakes up a sleeping VCPU.  It is more similar to PLE 
> (pause-loop exiting).
>
> Paolo

Yes, it is mostly to deal with vCPU that are not running because of PLE.

-Longman