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Date: Fri, 03 May 2013 11:54:34 +0100
From: Marc Zyngier <marc.zyngier@arm.com>
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To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Christopher Covington <cov@codeaurora.org>,
        Ian Campbell <Ian.Campbell@citrix.com>,
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Subject: Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen_ticks
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On 03/05/13 11:43, Stefano Stabellini wrote:
> On Thu, 2 May 2013, Christopher Covington wrote:
>>> So the virtual timer should appear to have been running even while time
>>> is being stolen and therefore stolen time needs to be accounted via some
>>> other means.
>>
>> Something that's not currently obvious to me is that given that the stolen
>> cycle accounting should be done, what makes the architected timer interrupt
>> handler the ideal place to do it?
> 
> That is a good question and I would appreciate suggestions to improve
> the patch.
> 
> Given that Xen x86 and ia64 does stolen time accounting from the timer
> interrupt handler:
> 
> arch/x86/xen/time.c:xen_timer_interrupt
> arch/ia64/kernel/time.c:timer_interrupt
> 
> and given that the arch_timer is the only timer used by Xen on ARM and
> that it includes a virt_timer that is made on purpose to be used by
> virtual machines, I thought that it might be a good place for it.
> 
> I also thought that doing it this way, KVM should be able to reuse the
> same hook.

Indeed. I just need to understand how time stealing works there ;-).

Now, KVM is not necessarily limited to arch_timers, and we've run KVM
using a QEMU-provided timer in the past. Can you think of a more generic
location for this hook? Possibly something that would satisfy the
requirements of other architectures while we're at it?

	M.
-- 
Jazz is not dead. It just smells funny...