Re: [RFC PATCH 0/5] Utilizing VMX preemption for timer virtualization

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, May 20, 2016 at 03:18:30PM -0300, Marcelo Tosatti wrote:
> On Thu, May 19, 2016 at 06:44:58PM -0700, Yunhong Jiang wrote:
> > The VMX-preemption timer is a feature on VMX, it counts down, from the
> > value loaded by VM entry, in VMX nonroot operation. When the timer
> > counts down to zero, it stops counting down and a VM exit occurs.
> > 
> > The VMX preemption timer for tsc deadline timer virtualization. The
> > VMX preemption timer is armed when the vCPU is running, and a VMExit
> > will happen if the virtual TSC deadline timer expires.
> > 
> > When the vCPU thread is scheduled out, the tsc deadline timer
> > virtualization will be switched to use the current solution, i.e. use
> > the timer for it. It's switched back to VMX preemption timer when the
> > vCPU thread is scheduled int.
> > 
> > This solution replace the complex OS's hrtimer system, and also the
> > host timer interrupt handling cost, with a preemption_timer VMexit. It
> > fits well for some NFV usage scenario, when the vCPU is bound to a
> > pCPU and the pCPU is isolated, or some similar scenarioes.
> > 
> > However, it possibly has impact if the vCPU thread is scheduled in/out
> > very frequently, because it switches from/to the hrtimer emulation a
> > lot. A module parameter is provided to turn it on or off.
> > 
> > Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
> 
> Hi Yunhong Jiang,
> 
> This adds cost to the VM-exit and VM-entry paths (additional
> instructions and i-cache pressure). Also it adds cost to
> kvm_sched_out.
> 
> What is the benefit the switch from external interrupt to VMX preemption 
> timer brings?

Do you have numbers for the improved interrupt latency in the guest ?