Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen_ticks

[Christopher Covington]

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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