linux-kernel - Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen

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Message-ID: <5183976A.6000703@arm.com>
Date:	Fri, 03 May 2013 11:54:34 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Christopher Covington <cov@...eaurora.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen_ticks

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...

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