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

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Message-ID: <5088EF85.4080002@de.ibm.com>
Date:	Thu, 25 Oct 2012 09:51:33 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Alexander Graf <agraf@...e.de>,
	Xiantao Zhang <xiantao.zhang@...el.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH 3/5] kvm: Directly account vtime to system on guest switch

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();
[...]

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