From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Hildenbrand <dahi@linux.vnet.ibm.com>
Subject: Re: [PATCH RFC 1/2] KVM: don't check for PF_VCPU when yielding
Date: Wed, 26 Nov 2014 10:23:19 +0100
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> This change is a trade-off.
> PRO: This patch would improve the case of preemption on s390. This is probably a corner case as most distros have preemption off anyway.
> CON: The downside is that kvm_vcpu_yield_to is called also from kvm_vcpu_on_spin. Here we want to avoid the scheduler overhead for a wrong decision.   

Won't most of that part be covered by:
	if (!ACCESS_ONCE(vcpu->preempted))

vcpu->preempted is only set when scheduled out involuntarily. It is cleared
when scheduled in. s390 sets it manually, to speed up waking up a vcpu.

So when our task is scheduled in (an PF_VCPU is set), this check will already
avoid scheduler overhead in kvm_vcpu_on_spin() or am I missing something?