Xen users - Oct 2011 - General questions about VGA passthrough

I have a question regarding VGA passthrough.
If I understood correctly, the only way to have VGA passthrough is hiding
the VGA from the Dom0 and giving full control of it to one guest. This
means, the guest should just take ownership of the device and start using it
(and therefore take control of the screen, too), right?
I read that one can dynamically hide PCI devices from the Dom0. What would
happen if one did that with a VGA device? For example, if I am using linux
with an active X server, what is likely to happen? In short, would it be
possible to "switch" the control of the video card from the Dom0 to
the
DomU, keeping the Dom0 "alive" (IE: no crashes), then giving the VGA
control
back to the Dom0 after stopping the guest?


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I will answer from my experiment using my nvidia graphic card.  The answer may
be different for ATI or Intel
> I have a question regarding VGA passthrough.
> If
 I understood correctly, the only way to have VGA passthrough is hiding 
the VGA from the Dom0 and giving full control of it to one guest. 

Yes
> This 
means, the guest should just take ownership of the device and start 
using it (and therefore take control of the screen, too), right?

Yes
> I read that one can dynamically hide PCI devices from the Dom0. 
Possible before starting the domU...I never tried to do that while the domU is
running.
> What 
would happen if one did that with a VGA device? 
> For example, if I am 
using linux with an active X server, what is likely to happen?
> In short,
 would it be possible to "switch" the control of the video card from
the
 Dom0 to the DomU, keeping the Dom0 "alive" 
> (IE: no crashes), then 
giving the VGA control back to the Dom0 after stopping the guest?

Giving control back to dom0 is not possible...Once you stop the domU (safe
shutdown), you need to restart the dom0

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2011/10/12 Francesco Cat <heartcollector87@gmail.com>
>  > What would happen if one did that with a VGA device?
>> > For example, if I am using linux with an active X server, what is
likely
>> to happen?
>> > In short, would it be possible to "switch" the control
of the video card
>> from the Dom0 to the DomU, keeping the Dom0 "alive"
>> > (IE: no crashes), then giving the VGA control back to the Dom0
after
>> stopping the guest?
>>
>
>  Giving control back to dom0 is not possible...Once you stop the domU
>> (safe shutdown), you need to restart the dom0
>>
>
Another question, this time it should be rather easy.
If I have 2 VGAs, and I manage to pass one to the DomU (Secondary adapter
passthrough should be doable for Xen >= 4.0 right? but that''s not
important)
can I still use a VNC connection to the DomU? I hope the answer is yes, but
I prefere asking since something strange might get in the way and cause
problems perhaps.


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Thanks you a lot, this tells me it''s probably better for me to stop
trying
to do strange stuff with xen and just resort to a headless server for my
other needs :P
I''m not an expert and even if I might end up setting exactly what I
need, it
might take me ages to achieve.


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