Re: [Xen-devel] [PATCH v6 05/11] pvqspinlock, x86: Allow unfair spinlock in a PV guest

[Waiman Long <waiman.long@xxxxxx>]

On 03/19/2014 06:07 AM, Paolo Bonzini wrote:
> Il 19/03/2014 04:15, Waiman Long ha scritto:
> > > > You should see the same values with the PV ticketlock. It is not clear
> > > > to me if this testing did include that variant of locks?
> > > Yes, PV is fine.  But up to this point of the series, we are concerned
> > > about spinlock performance when running on an overcommitted hypervisor
> > > that doesn't support PV spinlocks.
> > The unfair queue lock is designed in such a way that it will only be
> > activated when running in a PV guest or it won't be mergeable upstream.
> > So there must be some way to determine if it is running under a 
> > hypervisor.
> Exactly.  What you want is boot_cpu_has(X86_FEATURE_HYPERVISOR).
>
> Paolo
The unfair lock is to be enabled by boot time check, not just by the 
presence of a configuration macro during the build process in order to 
avoid using unfair lock on bare metal. Of course, Linux distros can 
modify this if that suits their need.
-Longman

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