Re: domU to dom0 switch question

[George Valzcig <gvalzcig@gmail.com>]

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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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