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

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 03/14/2014 05:57 PM, H. Sieger wrote:
> 1. I'm using distro packages only. In my first attempt I used Linux Mint
> 16 (based on Ubuntu 13.10), which is the latest stable release. This is
> what I'm currently running on my regular setup with Nvidia Quadro for
> domU and it works fine. It just doesn't work with the AMD 7770.
Can you double check that your initrd isn't loading the radeon 
framebuffer driver, and that the driver is blacklisted (along with 
fglrx) and neither of those are loaded at the point when the xen-pciback 
driver seizes the device?
The problem with ATI isn't on the initial passthrough. If you cannot 
even get that to work, then something else is going on.
> However, I am not a 100% sure if the Linux Mint install was completely
> up-to-date. I had installed LM16 some time ago, and I only did some
> package updates. I may have missed the kernel updates (which aren't
> applied automatically with updates and need to be done manually).
> The error 22 - invalid argument you reported is known to me. In fact, I
> replied to a similar bug report here
> <https://bugs.launchpad.net/ubuntu/+source/xen/+bug/1175155/comments/15>. Here
> a quote from the change log I keep with the VGA passthrough how-to:
> 30.04.2013 - warning re xen hypervisor release 4.1.3-3ubuntu1.5 upgrade
>
> As I said above, all my packages come from the distributions' stable
> branch, in this case Ubuntu 13.10 Saucy Salamander (who invents these
> names?) - I don't think Linux Mint changes anything in these packages.
I cam promise you that Xen 4.3.0 + kernel 3.9.9 in dom0 works fine for 
VGA passthrough on my system. If you are getting error 22, there is a 
good chance there is a regression in the distro package where they 
missed a patch (or re-introduced a broken one).
> 2. The second trial with Linux Mint 13 (based on Ubuntu 12.04 LTS
> Precise Pangolin), using the xm toolstack, worked as it should. I was
> able to shutdown and reboot the domU multiple times without any issue -
> no slowdown, no instability, solid as a rock. I did not try the xl
> toolstack with that release, as xl then was still in the diapers.
>
> By the way, I dumped virt-manager because it causes more troubles than
> it does good. I also searched in vain for a config file.
I have up on virt-manager a very long time ago. Anybody needing to do 
something like VGA passthrough and expecting to be able to do so with a 
pretty GUI is going to be out of luck for a few years yet. VGA 
passthrough seems to be strictly for people who know what they are doing 
or can afford to pay a 3rd party company to set it all up for them.
> I believe that Xen 4.1.2 with xm toolstack and an old 3.2 kernel is
> still the best way to get VGA passthrough running. The migration to xl
> introduced some issues, so did some of the security patches (the XSA
> patches). So, when you got a working system, don't upgrade if you can
> help it.
You still haven't clarified whether you are able to reboot the domU with 
the ATI card and still have it working without performance degradation, 
BSODs, the card not showing up at all, or crashing the host.
The latter you can probably avoid if your PCIe bridges support ACS so 
the IOMMU forcibly prevents the card from trampling all over something 
else's memory areas, but it won't help with general stability - it is 
only there to protect the host and other guests from a rogue guest.
Gordan

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