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

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Message-ID: <518A3B81.3080308@codeaurora.org>
Date:	Wed, 08 May 2013 07:48:17 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Ian Campbell <Ian.Campbell@...rix.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

On 05/08/2013 07:19 AM, Stefano Stabellini wrote:
> On Tue, 7 May 2013, Christopher Covington wrote:
>> 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?
> 
> Although it might be possible to hide the real time flow from the VM, it
> is inadvisable: what would happen when the VM needs to deal with a real
> hardware device? Or just send packets over the network?  This is why it
> is much safer and more reliable to expose the stolen ticks to the VM.

One would probably have to emulate all the hardware. I don't mean to imply
that I think this is useful for everyday virtualization, but I speculate that
such an approach might enable the analysis of kernels infected with
VM-detecting malware or other niche use-cases.

Christopher

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