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

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Date:	Fri, 3 May 2013 11:43:23 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Christopher Covington <cov@...eaurora.org>
CC:	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>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"will.deacon@....com" <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 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.
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