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Thanks again!

The thing about this bug is that it only happens with the xl toolstack, the xm toolstack works fine, unless you run Windows 8.1 in the guest (Windows 8 runs fine with xm, so does Windows 7).

The way I see it:
1. AMD graphics cards - use xm toolstack (unless for Windows 8.1 guests)
2. Nvidia Quadro graphics cards - use xl or xm
3. Windows 8.1 guests - require xl toolstack

How come xm usually works with AMD cards, whereas xl doesn't?

Considering the growing popularity of VGA passthrough and the noise on that issue, perhaps it will get some attention soon? It seems that kvm is taking the lead on that.

On Wednesday, March 12, 2014 2:33 PM, Gordan Bobic <gordan@xxxxxxxxxx> wrote:

On 03/12/2014 09:25 AM, H. Sieger wrote:
> Thanks for the detailed reply, Gordon. Very helpful!
>
> Can someone tell me which Xen release implements reinitializing passed
> through devices to fix the AMD driver bug when rebooting the domU?

It's definitely not in 4.3.x (what I'm running). I don't know if it made
it into the 4.4.x release.

> I believe I read somewhere that perhaps a new AMD graphics card driver
> might solve the issue. Does anybody know about that?

It could - but that would require ATI to fix their driver - which given
how otherwise broken their drivers are in various ways, I wouldn't hold
my breath for.

I tried getting an ATI based solution very hard, but in the end I failed
to achieve anything workable. I got a Quadro 2000 for testing, which
"just worked". So I cut my losses, traded in my ATIs for Nvidias and
have been using rather than troubleshooting the system since. From there
on I went with modifying GTS450 cards into Quadro 2000s, then went on to
GTX470 and 480, modifying them to Quadro 5000 and 6000 respectively,
because it was easy and they could be soft modded. More recently I've
been using GTX680/GTX690 (Tesla K10 / Grid K2) and GTX780Ti (Quadro
K6000) cards but that requires physically modifying the card.

Or, if your decompiling and assembly-foo is strong, and you are that way
inclined you could void your EULA and modify the binary driver to neuter
it's decision to not initialize the card if it's device ID isn't on the
whitelist. Something similar was also done to enable SLI across
dissimilar cards (google for DifferentSLI).

> It's quite strange that on one side AMD supports VGA passthrough (a lot
> more than Nvidia does), but on the other side they haven't fixed their
> Windows driver to properly reset the card (at least that is how I
> understand it).

AMD have always been very good at making positive noises about
supporting various open source things, but very little has ever come of
it. For example, they support GCC, whereas Intel has the closed-source
commercial ICC (or whatever they call it nowdays). While AMD's efforts
are ideologically commendable, Intel's compiler is still, sadly way, way
ahead in features and performance of the code it produces - including
when said code runs on AMD processors (although Intel did get some very
bad publicity back when they released a version of ICC generated code
that detected the CPU ID and if it wasn't Intel it disabled
vectorization which is where most of the speed-up comes from, but this
doesn't happen any more, they are happy to take your money for the
compiler even if you are going to run your code on an AMD CPU).

> This domU reboot issue with xl has been around for some time and I was
> hoping that some fix has trickled into a recent Xen release or AMD
> driver release.

It has been around for years, but then again so have a lot of other
issues with ATI drivers. In the end it won't be fixed in the driver,
it'll be fixed by Xen adding bus reset to the list of reset methods.

Unfortunately, the choice is between two solutions, neither of which is
ideal. But one of the options is a lot more workable.

Gordan

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