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

[Waiman Long]

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.

-Longman
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