Re: [RFC PATCH] KVM: x86: Skip request checking branches in vcpu_enter_guest() more effectively

[Takuya Yoshikawa <takuya.yoshikawa@gmail.com>]

On Mon, 24 Sep 2012 09:16:12 +0200
Gleb Natapov <gleb@redhat.com> wrote:

> Yes, for guests that do not enable steal time KVM_REQ_STEAL_UPDATE
> should be never set, but currently it is. The patch (not tested) should
> fix this.

Thinking a bit more about KVM_REQ_STEAL_UPDATE...

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 901ad00..01572f5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1544,6 +1544,8 @@ static void accumulate_steal_time(struct kvm_vcpu *vcpu)
>  	delta = current->sched_info.run_delay - vcpu->arch.st.last_steal;
>  	vcpu->arch.st.last_steal = current->sched_info.run_delay;
>  	vcpu->arch.st.accum_steal = delta;
> +
> +	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>  }

Even without this flag, we know when we should record the steal time:
that's when vcpu->arch.st.accum_steal != 0.

>  
>  static void record_steal_time(struct kvm_vcpu *vcpu)

So we can live without KVM_REQ_STEAL_UPDATE and make record_steal_time()
do the job when vcpu->arch.st.accum_steal != 0 so that it can be called
unconditionally in vcpu_enter_guest().

Why didn't we do that?  The only reason I can think of is we wanted to
eliminate that check for the zero-requests case.

But does such a micro-optimization really help us?

Thanks,
	Takuya