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

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Date:	Thu, 25 Oct 2012 09:59:50 +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 09:56, Frederic Weisbecker wrote:
[...]
>>> * 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?

Yes. We added the local_irq_disable code just for the vtime accounting.




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