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Re: [Xen-devel] [Xen-users] Xen 4.7.0 graphics pass through problems (Win10, AMD Radeon)

On 2016-09-14 16:17, George Dunlap wrote:
On Wed, Sep 14, 2016 at 3:04 PM, Peter Milesson <miles@xxxxxxxx> wrote:

On 2016-09-14 14:50, Jan Beulich wrote:
On 14.09.16 at 13:34, <george.dunlap@xxxxxxxxxx> wrote:
   On 2016-09-09 09:51, Peter Milesson wrote:
I've been using Xen for 3 years, starting out with 4.2, and I've been
upgrading regularly, and installed 4.7.0 today.

I'm using Windows 10, 64-bit with PCI pass through in a VM with
PV-drivers, using a AMD Radeon HD6450 card (AMD graphics cards don't
seem to
need graphics pass through)

Previously (up till Xen 4.6.3), the graphics output has displayed some
shorter lines, a bit like thin coarse snow, when watching videos. The
distortions stayed within the movie, and was tolerable.

After upgrade to Xen 4.7.0, the video performance is seriously ugly.
for example, I open cnn.com and move the mouse pointer up and down over
photos, which creates a bunch of flickering lines over the display. The
with moving content in Youtube for example. Or opening a Cygwin
window and scrolling through a file. Terrible.

Nothing else has changed, only the Xen version.

On Fri, Sep 9, 2016 at 10:35 AM, Peter Milesson <miles@xxxxxxxx> wrote:
Hi again,

I've been playing around a bit more.

It seems one of the problems is that I passed a couple of PCI-devices
controllers) to the VM, beside the graphics card.

When I pass the USB-devices individually in the VM configuration file
(usbdevice=['tablet','host:1.4','host:1:9','host:x.y']), the display
problems seem to be more or less gone.
Jan / Andy,

Can you think of anything that's changed between 4.6 and 4.7 that
would cause the performance problems he's describing when both a video
card and a usb controller are passed through, but not when only the
video card is passed through?
Not really, no. Peter - are there any indications of problems in
one or more of the logs (Xen, xl, qemu)? Did you try running a
debug build of all Xen components?


Hi Jan,

No, I did not try a debug build. However, the system seems fairly stable
now, only the display driver crashing once, or twice a day. That did occur
with Xen 4.6.3 also, but maybe once a week.
Well crashing twice a day isn't what most people would call "stable".
:-)  In any case, I was trying to debug the issue you had with passing
through both the video card and the usb controller -- that is
obviously an important configuration, but if there's a bug in that
configuration it probably affects a large number of wider
configurations as well.

Looking at the log files, I
cannot see anything particular that stands out, except for the domU log.

The domU log file qemu-dm-<Name>.log started to contain the following lines
after upgrading to 4.7:

[00:06.0] xen_pt_bar_read: Error: Should not read BAR through QEMU.
[00:06.0] xen_pt_bar_write: Error: Should not write BAR through QEMU.

There are only a couple of lines with write, the rest with read. I don't
know what significance it's got, but there are around 350 lines every day.
Yeah, those definitely look important.  This is with just the video
card passed through, or with both passed through?

Hi George,

This is with both the USB controller, and the graphics card.

Best regards,


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