Messages in this thread | | | Date | Thu, 25 Oct 2012 09:56:58 +0200 | Subject | Re: [PATCH 3/5] kvm: Directly account vtime to system on guest switch | From | Frederic Weisbecker <> |
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2012/10/25 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(); > [...] >
Ah ok. Hmm I still need to keep it irqsafe for the other archs though, as it is currently with vtime_account(). So perhaps I can remove your local_irq_disable there and use vtime_account_system_irqsafe() instead?
thanks.
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