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

[Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>]

* 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?

> 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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