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Subject: Re: [PATCH 2/3] kvm hypervisor : Add hypercalls to support
 pv-ticketlock
From: Peter Zijlstra <peterz@infradead.org>
To: vatsa@linux.vnet.ibm.com
Cc: Jeremy Fitzhardinge <jeremy@goop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Date: Wed, 19 Jan 2011 18:50:15 +0100
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On Wed, 2011-01-19 at 22:53 +0530, Srivatsa Vaddagiri wrote:
> On Wed, Jan 19, 2011 at 10:42:39PM +0530, Srivatsa Vaddagiri wrote:
> > Add two hypercalls to KVM hypervisor to support pv-ticketlocks.
> > 
> > KVM_HC_WAIT_FOR_KICK blocks the calling vcpu until another vcpu kicks it or it
> > is woken up because of an event like interrupt.
> 
> One possibility is to extend this hypercall to do a directed yield as well, 
> which needs some more thought. Another issue that needs to be resolved with
> pv-ticketlocks is the impact on intra-VM fairness. A guest experiencing heavy
> contention can keep yielding cpu, allowing other VMs to get more time than they
> deserve.

No, yield sucks, if you know the owner you can do actual PI.