Re: [PATCH 3/5] kvm: Directly account vtime to system on guest switch

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 25/10/12 02:51, Frederic Weisbecker wrote:
> Switching to or from guest context is done on ioctl context.
> So by the time we call kvm_guest_enter() or kvm_guest_exit()
> we know we are not running the idle task.
> 
> As a result, we can directly account the cputime using
> vtime_account_system_irqsafe().
> 
> There are two good reasons to do this:
> 
> * We avoid some useless checks on guest switch. It optimizes
> a bit this fast path.
> 
> * In the case of CONFIG_IRQ_TIME_ACCOUNTING, calling vtime_account()
> checks for irq time to account. This is pointless since we know
> we are not in an irq on guest switch. This is wasting cpu cycles
> for no good reason. vtime_account_system() OTOH is a no-op in
> this config option.
> 
> * s390 doesn't disable irqs in its implementation of vtime_account().
> If vtime_account() in kvm races with an irq, the pending time might
> be accounted twice. With vtime_account_system_irqsafe() we are protected.

We disable irqs before we call kvm_guest_enter/exit, see kvm-s390.c:


[...]
	if (!kvm_is_ucontrol(vcpu->kvm))
	        kvm_s390_deliver_pending_interrupts(vcpu);
	vcpu->arch.sie_block->icptcode = 0;
	local_irq_disable();
	kvm_guest_enter();
	local_irq_enable();
[...]