Xen devel - Mar 2010 - switching passthrough GFX between several domains

Hello, everybody.

I''ve some success with gfx passthough and successfully run Windows XP 
with real nvidia 8400gs card inside.
Now, there are 2 running HVM domains in my configuration, and I want to 
switch real gfx between them, i.e allow only one VM to have access to 
the graphic card.

Firstly, I tried to switch real GFX by suspending one of the VM and 
giving the gfx to another VM.
But I found out that it''s impossible to suspend VM with assigned PCI 
device:

  "Migration not permitted with assigned PCI device"

I agree that migration to another hardware is a problem, but why don''t
suspend/resume implemented for the same hardware?


I tried to unplug gfx card manually by doing this:

#xm pci-detach winxp 04:00.0

But with no success. Guest nvidia''s driver said that:

Device NVIDIA GeForce 8400 GS couldn''t stopped right now. Try to stop
it
later.

So, I couldn''t release attached pci device.

Is any way to solve the task of switching gfx between different HVM domain?
I''m not sure, but what happened if I take off the graphic card manually
by
* save the contents of the memory-mapped regions and io regions.
* forbit access to configuration space  from QEMU
* unmap access to the real MMIOs, PIOs from inside the hypervisor.

And then I''ll reset this stuff and reinitialize them for the new VM. 
Will it work at all?
Of course, there will be a lot synchronization and transaction problems. 
But is it possible at all?


Thanks for your attention.

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