Re: domU to dom0 switch question

[George Valzcig <gvalzcig@gmail.com>]

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Hello,
Please anyone help my question?
Thanks

On Fri, Apr 2, 2010 at 10:28 PM, George Valzcig <gvalzcig@gmail.com> wrote:

>
>
> On Fri, Apr 2, 2010 at 10:13 PM, Jeremy Fitzhardinge <jeremy@goop.org>wrote:
>
>> On 04/02/2010 04:02 PM, George Valzcig wrote:
>>
>>> I have two questions on between a switch one dom to another dom in Xen. I
>>> hope this is the right forum to ask these questions ...
>>>
>>> 1) other than I / O, there is no reason to switch from one domU to dom0
>>> (memory page remapping etc)? If I have a 1 CPU in my physical machine and I
>>> run a domU with while loop that does nothing but use CPU, there is no reason
>>> to switch to dom0?
>>> 2) In Xen PV is the hyper call entry page in the address space of the
>>> domU - so call to a mapped hyper call not cause context switch - yes?
>>>
>>
>> I don't follow your questions.
>>
>> "Dom0" is the domain started by Xen automatically at boot time, which is
>> privileged and is typically used for its hardware device drivers and
>> systemwide administrative functions.  "DomU" is a less privileged "guest"
>> domain, which may be either paravirtualized (PV) or using hardware
>> virtualization features (HVM).
>>
>> Note that "dom0" and "domU" are just roles; a single (pvops Linux) kernel
>> image can be used unchanged as dom0, PV domU or HVM domU.
>>
>> You seem to be using the terms "dom0" and "domU" in some other way, so I
>> think you need to clarify the question before you get the answer you're
>> looking for.
>>
>>
> Thanks for reply. Sorry for unclear question.
>
> I read from the Xen paper dom0 is used for I/O backend driver - you also
> explain it - thanks. So if some domU wants to I/O, *and only one physical
> CPU*, domU gets unscheduled - dom0 scheduled - yes?
>
> My question 1 is, apart from I/O backend driver, is dom0 used for anything
> else? I assume application running on domU no I/O. Any other reason why domU
> will require the help of dom0.
>
> My question 2 is, domU requests some service (like page table update) using
> hypercall. Will this (executing the hypercall and related code) need dom0?
>
> My questions because, I want to understand the increased number of context
> switches and TLB cost (as reported in the paper "The cost of virtualization"
> by Ulrich Drepper) in the case of Xen.
>
> Thanks.
>
>
>
>>    J
>>
>
>

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