From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT
 instructions (v2)
Date: Thu, 2 Dec 2010 18:40:16 -0200
Message-ID: <20101202204016.GB31316@amt.cnet>
References: <1291298357-5695-1-git-send-email-aliguori@us.ibm.com>
 <20101202191416.GQ10050@sequoia.sous-sol.org>
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Cc: Anthony Liguori <aliguori@us.ibm.com>, kvm@vger.kernel.org,
	Avi Kivity <avi@redhat.com>,
	Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
To: Chris Wright <chrisw@sous-sol.org>
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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.