Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions (v4)

[Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>]

On Mon, Dec 06, 2010 at 03:27:18PM +0200, Avi Kivity wrote:
> >A vcpu could be idle not just because of lack of work, but also because its
> >waiting on IO completion. Normally idle vcpus that yield would allow their
> >companion threads to run and possibly finish pending IO work faster. Now that
> >idle vcpu won't yield, it would cause overall cpu cycle demand of VMs to go up
> >(100% demand from VM's idle/not-idle vcpus + whatever their companion threads
> >demand) not to mention any impact on IO latencies. Not sure how much of an issue
> >this will be in practice, but something to keep in mind when we test!
> 
> It will be an issue.  Anything that is latency sensitive will
> suffer, since the scheduler won't prioritize completions (at least
> in vcpu threads).  But that only affects the average case, not the
> worst case.

Yeah - some testing will tell us how much of an issue this is for various
workloads.

> >Also, just curious how this would work for idle vcpus that use mwait rather
> >than hlt.
> 
> We don't expose mwait to the guest (emulating mwait is very expensive).

We seem to be queuing an exception upon mwait (EXIT_REASON_MWAIT_INSTRUCTION
being handled by a handle_invalid_op()). Does that kill the guest?

- vatsa