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

["H. Peter Anvin" <hpa@zytor.com>]

On 02/19/2014 11:24 AM, 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.
> 

The case where we run native on a system with CONFIG_PARAVIRT enabled
DOES matter.

	-hpa