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

[Marcelo Tosatti <mtosatti@redhat.com>]

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?