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Re: [Xen-devel] FW: PCI and VGA Passthrough regressions on Xen 4.4.1 vs 4.3.2

On 2014-10-13 19:41, Zir Blazer wrote:
Some further tests:

0 - Replacing the hvmloader from Xen 4.4.1 with the one from 4.3.2
(File /usr/lib/xen/boot/hvmloader) produces same results with the
Sound Card. So it is not the hvmloader, either.
A thing with I think that COULD be, assuming it is not Xen itself, is
that on Xen 4.3.x the Linux Sound Card Driver starts and grab the
device before xen-pciback, so it effectively initializes it before
xen-pciback makes it available for DomU use, which is the workaround
that allows me to get it working on 4.4.1. As I don't know the order
that they currently start I don't know if that's possible, but I
suppose that it could be tested by blacklisting the Sound Card Drivers
in Xen 4.3.2, which should produce the same horrible sound that
happens on Xen 4.4.1 if that were the case.

If you are planning to pass PCI hardware to domU, you really
should blacklist the drivers for said hardware in dom0, or
if you have other hardware accessible with the same driver in
dom0, blacklist the driver anyway, make sure you configure
xen-pciback to take charge of specific devices (create a
module config file in /etc/modprobe.d/) and make sure it
loads before the hardware drivers (put a script to load
xen-pciback in /etc/sysconfig/modules, followed by loading
the drivers you blacklisted for dom0 access to other similar

2 - After checking some VGA Passthrough guides, many said to blacklist
the radeon Driver, which after checking with lsmod | grep radeon I had
running. So I added the modprobe.blacklist=radeon line for the Kernel
in the Boot Loader config, which seemed quite straightforward compared
to remaking the initramfs image. This effectively seemed to remove it,
cause lsmod | grep radeon gave no results anymore. If using xl
pci-assignable-add 01:00.0, there were no freezes or anything
abnormal, just a warning than it was unbinded with no Drivers claiming

Yup, pretty much equivalent to what I said above.

Also, you may be interesed in a theory I have about the famous
"performance degradation" with Radeons:

That _could_ be a part of the problem but there is at least
something else broken going on because being locked into
low power mode wouldn't explain various display corruption
that also occurs.


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