domU to dom0 switch question

domU to dom0 switch question

[George Valzcig]

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Hi,
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?

(Sorry for English not my language)
Thanks.

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Re: domU to dom0 switch question

[Jeremy Fitzhardinge]

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.

     J

Re: domU to dom0 switch question

[George Valzcig]

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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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Re: domU to dom0 switch question

[George Valzcig]

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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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Re: domU to dom0 switch question

[Jeremy Fitzhardinge]

On 04/02/2010 07:28 PM, George Valzcig wrote:
> 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?

Yes.  From Xen's perspective, dom0 is just another domain, so it 
schedules all vcpus the same way; if there's contention for pcpus, then 
it needs to context switch between multiple vcpus according to its 
scheduling algorithm.

> 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.

Not really.  Once a domain has started, it is independent of dom0 unless 
it specifically makes use of its services.  An hvm domain using a stub 
domain won't make any direct use of dom0 - all its IO requests will be 
proxied via the stub domain.

> My question 2 is, domU requests some service (like page table update) 
> using hypercall. Will this (executing the hypercall and related code) 
> need dom0?

No.  Xen handles all hypercalls directly (though some of them may 
involve another domain).


     J