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Re: [Xen-devel] [PATCH RFC V4 5/5] Documentation/kvm : Add documentation on Hypercalls and features used for PV spinlock

On Tue, Jan 17, 2012 at 07:58:18PM +0530, Srivatsa Vaddagiri wrote:
> * Gleb Natapov <gleb@xxxxxxxxxx> [2012-01-17 15:20:51]:
> > > Having the hypercall makes the intent of vcpu (to sleep on a kick) clear 
> > > to 
> > > hypervisor vs assuming that because of a trapped HLT instruction (which
> > > will anyway won't work when yield_on_hlt=0).
> > > 
> > The purpose of yield_on_hlt=0 is to allow VCPU to occupy CPU for the
> > entire time slice no mater what. I do not think disabling yield on HLT
> > is even make sense in CPU oversubscribe scenario.
> Yes, so is there any real use for yield_on_hlt=0? I believe Anthony
> initially added it as a way to implement CPU bandwidth capping for VMs,
> which would ensure that busy VMs don't eat into cycles meant for a idle
> VM. Now that we have proper support in scheduler for CPU bandwidth capping, 
> is 
> there any real world use for yield_on_hlt=0? If not, deprecate it?
I was against adding it in the first place, so if IBM no longer needs it
I am for removing it ASAP.

> > Now if you'll call
> > KVM_HC_WAIT_FOR_KICK instead of HLT you will effectively ignore
> > yield_on_hlt=0 setting.
> I guess that depends on what we do in KVM_HC_WAIT_FOR_KICK. If we do
> yield_to() rather than sleep, it should minimize how much cycles vcpu gives 
> away
> to a competing VM (which seems to be the biggest purpose why one may
> want to set yield_on_hlt=0).
> > This is like having PV HLT that does not obey
> > VMX exit control setting.
> - vatsa


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