Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

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

On 02/19/2014 02:48 PM, Peter Zijlstra wrote:
> On Wed, Feb 19, 2014 at 02:24:49PM -0500, Waiman Long wrote:
>> On 02/19/2014 03:51 AM, Peter Zijlstra wrote:
>>> On Tue, Feb 18, 2014 at 07:42:20PM -0500, Waiman Long wrote:
>>>> On 02/18/2014 04:28 PM, Peter Zijlstra wrote:
>>>>> On Tue, Feb 18, 2014 at 02:30:12PM -0500, Waiman Long wrote:
>>>>>> I will start looking at how to make it work with paravirt. Hopefully, it
>>>>>> won't take too long.
>>>>> The cheap way out is to simply switch to the test-and-set spinlock on
>>>>> whatever X86_FEATURE_ indicates a guest I suppose.
>>>> I don't think there is X86_FEATURE flag that indicates running in a guest.
>>>> In fact, a guest should never find out if it is running virtualized.
>>> No it very much should; how else is paravirt ever going to work?
>> We do have a CONFIG_PARAVIRT macro that turns on or off PV support. The
>> queue spinlock can be easily changed into an unfair lock which allows lock
>> stealing. We could have a config option to make it unfair in the PARAVIRT
>> environment, but I don't think Linus like the idea of an unfair lock.
> No; a guest is very much aware of paravirt. See for example the
> static_key_false(&paravirt_ticketlocks_enabled). It would be impossible
> to set that branch if you never knew you were a guest.

Yes, that is true for paravirt, but I was talking about virtualization 
in general.

-Longman