Re: [Xen-users] VGA passthrough with Xen 4.3 and xl toolstack - performance degradation resolved?

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 03/13/2014 07:06 AM, H. Sieger wrote:
> I can't (and won't) argue with trying it out. However, the issue here
> (performance loss/instability after domU shutdown/reboot) has nothing to
> do with blacklisting the graphics driver.
On the contrary, it is related because it affects the state of the card. 
If the state of the ATI card has been "tainted" by being touched by the 
driver, once you detach it from the driver and to pciback it will react 
the same as it does after a reboot of the domU it was passed to.
> Blacklisting the graphics
> driver(s) is not only necessary for AMD/ATI graphics cards, but also for
> Nvidia cards.
As I said before, with an Nvidia card, I can move the card around 
between domains without rebooting the machine. I haven't tried it in a 
few months, but I had been able to shut down the domU with the card, 
detach it from pciback, re-load the driver in dom0, use the card in 
dom0, remove the nvidia driver and re-attach the card to pciback, and 
then fire up another domU with the same card. So blacklisting is not 
strictly necessary because Nvidia cards don't seem to be affected by 
being in a pre-initialized state.
> This has to do with how and when pciback (or pci-stub) is
> trying to grab the graphics card - if that happens after a graphics
> driver took possession, bad luck (according to my experience).
In my experience this can be made to work. Off the top of my head, you 
can detach the card from it's current driver using something like:
echo $device_id > /sys/bus/pci/drivers/nvidia/unbind

and then attack it to pciback using

echo $device_id > /sys/bus/pci/drivers/pciback/bind

(don't quote me on the exact incantation, but that is the gist of it).

Or you could use xl pci-assignable-add which essentially does the same 
thing.
I do recall this was more problematic before (maybe around xen 4.1/4.2 
days), but I'm sure last time I tried it did work on my setup.
> Now that you mention pci-stub, have you tried pciback? Perhaps the whole
> issue is pci-stub related?
When I said pci-stub I was of course referring to xen-pciback. pci-stub 
is what the equivalent KVM driver is called.


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