Re: [patch 2/2] KVM: x86: add option to advance tscdeadline hrtimer expiration

Rik van Riel
2014-12-10T17:20:00Z

On 12/10/2014 12:06 PM, Marcelo Tosatti wrote:
> For the hrtimer which emulates the tscdeadline timer in the guest,
> add an option to advance expiration, and busy spin on VM-entry waiting
> for the actual expiration time to elapse.
>
> This allows achieving low latencies in cyclictest (or any scenario
> which requires strict timing regarding timer expiration).
>
> Reduces cyclictest avg latency by 50%.
>
> Note: this option requires tuning to find the appropriate value
> for a particular hardware/guest combination. One method is to measure the
> average delay between apic_timer_fn and VM-entry.
> Another method is to start with 1000ns, and increase the value
> in say 500ns increments until avg cyclictest numbers stop decreasing.

It would be good to document how this is used, in the changelog.