Re: [Xen-devel] questions about xen device pass-through.

[Jean Guyader <jean.guyader@xxxxxxxxx>]

On 17 August 2011 20:55, Dushmanta Mohapatra
<dushmanta.mohapatra@xxxxxxxxx> wrote:
> Hello,
>
> I have some doubts/questions regarding device passthrough support in Xen.
> I will really appreciate some help/suggestions.
>
> (I am using a Intel Core i5 vPro based Lenovo thinkpad T520 (Intel graphics
> adapter).
> I am running Xen 4.1.1 and PVOPS based Linux 2.6.32.41 kernel as Dom0. I use
> Windows XP
> and Windows 7 as my HVM Domus. lspci details are attached.)
>
> I am trying to achieve the following: Initially allow
> direct access to the primary devices (graphics/ethernet/wireless/USB) to an
> HVM DomU. After wards,
> depending on the need I want to re-assign the direct access of these devices
> back to Dom0 and
> have the DomU access the devices in a virtual manner.
>
> So my question is: Is it possible to do it? Is it possible for VGA/
> graphics?
>
> I have tried a few things and following is what I am being able to do:
>
> I am being able to provide direct access to devices
> (wireless/ethernet/graphics etc) to HVM DomUs.
> And also I am being able to dynamically change the direct access to these
> devices between
> the various HVM DomUs (tested with WinXP and Windows 7) in case of
> ethernet/USB etc. I am being
> able to detach it from one DomU and attach it to another. (This is not
> working with VGA direct access
> though.)
>
> But I am not being able to reattach these devices back to Dom0. I have tried
> the sysfs way and initial
> boot time passthrough.
>
> I have also tried the stubdomain way, but when I do not hide the devices
> from Dom0 (at boot time) and later try to assign them to stubdomain, my Dom0
> gets rebooted automatically. I am following
> the steps mentioned in:
> http://mirror.choon.net/xen/xen-unstable.hg/docs/misc/vtd.txt
>
> So could some one  tell me if these behaviors are expected or
> if I might be doing something wrong.
>
> I will really appreciate any help. Please let me know if more information is
> needed from my side.
>

Hi,

If you detach a pass through device from a domU it gets reassigned to
dom0 automatically.
The problem you are having is that pciback in dom0 still own the
device that why it looks like
the device isn't working in dom0.

You'll have to manually unbind the device from picback and bind it to
another driver in dom0.
That can be done through the sysfs driver nodes "bind" and "unbind".

Jean

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