Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions (v2)

Marcelo Tosatti
2010-12-02T20:41:18Z

On Thu, Dec 02, 2010 at 11:14:16AM -0800, Chris Wright wrote:
> * Anthony Liguori (aliguori@us.ibm.com) wrote:
> > In certain use-cases, we want to allocate guests fixed time slices where idle
> > guest cycles leave the machine idling. There are many approaches to achieve
> > this but the most direct is to simply avoid trapping the HLT instruction which
> > lets the guest directly execute the instruction putting the processor to sleep.
>
> I like the idea, esp to keep from burning power.
>
> > Introduce this as a module-level option for kvm-vmx.ko since if you do this
> > for one guest, you probably want to do it for all. A similar option is possible
> > for AMD but I don't have easy access to AMD test hardware.
>
> Perhaps it should be a VM level option. And then invert the notion.
> Create one idle domain w/out hlt trap. Give that VM a vcpu per pcpu
> (pin in place probably). And have that VM do nothing other than hlt.
> Then it's always runnable according to scheduler, and can "consume" the
> extra work that CFS wants to give away.
>
> What do you think?
>
> thanks,
> -chris

Consuming the timeslice outside guest mode is less intrusive and easier
to replace. Something like this should work?

if (vcpu->arch.mp_state == KVM_MP_STATE_HALTED) {
while (!need_resched())
default_idle();
}

But you agree this is no KVM business.