Re: [PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline

[Paolo Bonzini]

On 09/10/21 04:12, Sean Christopherson wrote:
> Calculate the halt-polling "stop" time using "cur" instead of redoing
> ktime_get().  In the happy case where hardware correctly predicts
> do_halt_poll, "cur" is only a few cycles old.  And if the branch is
> mispredicted, arguably that extra latency should count toward the
> halt-polling time.
>
> In all likelihood, the numbers involved are in the noise and either
> approach is perfectly ok.

Using "start" makes the change even more obvious, so:

    Calculate the halt-polling "stop" time using "start" instead of redoing
    ktime_get().  In practice, the numbers involved are in the noise (e.g.,
    in the happy case where hardware correctly predicts do_halt_poll and
    there are no interrupts, "start" is probably only a few cycles old)
    and either approach is perfectly ok.  But it's more precise to count
    any extra latency toward the halt-polling time.

Paolo