Xen 4.3 / 4.4 - concurrent APIs, VGA Passthru

[Georg Bege <therion@ninth-art.de>]

Hello folks,

since a couple of days Im intensively trying to get around with Xen -
so far I've been successful.
Xen is really complex, I've just figured out that there are two APIs -
one which is obsolete (xend/xm) the other one which is newer/current (XL).

In fact I had already read about it, but what I did not know was that
the XL API
is not able to utilize VGA Passthru, why is this?
When I try it, exacly same box, hardware, kernel everything - upon xl
create ...
I get an obscure error message:
libxl: error: libxl_pci.c:990:libxl__device_pci_reset: The kernel
doesn't support reset from sysfs for PCI device 0000:03:00.0

I was recompiling my kernel like 6 times until I figured out, there is
no option I did forget - its not about the Linux Kernel.
(I was suprised before I had it running prior the upgrade to Xen 4.4.0
(Gentoo repos))
So I did back downgrade to 4.3.2 in order to get xend/xm again and voila
- with xm create I do not get this messages.

So all I want to know is, why is this?
What is the difference - is it simply not implemented yet? Will it be
implemented?
Or am I still doing something wrong?

And can I use both, lets say for VMs where I do not need passthru, I use
XL tools - for the other VM I use XM? - As far as I know xm/xl produce
the same outputs literally.

regards,
Georg