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

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Message-ID: <51892933.7090405@codeaurora.org>
Date:	Tue, 07 May 2013 12:17:55 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	"catalin.marinas@....com" <catalin.marinas@....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

Hi Konrad,

On 05/06/2013 10:35 AM, Konrad Rzeszutek Wilk wrote:
>>> e.g. if a VCPU sets a timer for NOW+5, but 3 are stolen in the middle it
>>> would not make sense (from the guests PoV) for NOW'==NOW+2 at the point
>>> where the timer goes off. Nor does it make sense to require that the
>>> guest actually be running for 5 before injecting the timer because that
>>> would mean real time elapsed time for the timer would be 5+3 in the case
>>> where 3 are stolen.
>>
>> This is a bit of an aside, but I think that hiding time spent at higher
>> privilege levels can be a quite sensible approach to timekeeping in a
>> virtualized environment, but I understand that it's not the approach taken
>> with Xen, and as you pointed out above, adjusting the Virtual Offset Register
>> by itself isn't enough to implement that approach.
> 
> This is the approach taken by Xen and KVM. Look in CONFIG_PARAVIRT_CLOCK for
> implementation. In the user-space, the entry in 'top' of "stolen" (%st)
> is for this exact value.

I may have been unclear with my terms, sorry. When I refer to time being
"hidden", I mean that kernel level software (supervisor mode, EL1) cannot
detect the passage of that time at all. I don't know whether this would really
work, but I imagine one might be able to get close with the current
virtualization facilities for ARM.

Am I correct in interpreting that what you're referring to is the deployment
of paravirtualization code that ensures (observable) "stolen" time is factored
into kernel decision-making?

Thanks,
Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
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