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

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

On Sat, Dec 04, 2010 at 08:43:21AM -0600, Anthony Liguori 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.

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!

Also, just curious how this would work for idle vcpus that use mwait rather 
than hlt.

- vatsa