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

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 03/12/2014 05:51 PM, H. Sieger wrote:
> I'm referring to secondary passthrough. What you are saying here is
> quite a surprise to me. A number of people have reported successful VGA
> passthrough using ATI cards, see Post your Passmark results of your
> Windows VM <http://forums.linuxmint.com/viewtopic.php?f=225&t=153482>
> and HOW-TO make dual-boot obsolete using XEN VGA passthrough
> <http://forums.linuxmint.com/viewtopic.php?f=42&t=112013>. Since I wrote
> the how-to more than a year and a half ago, with regular updates, I
> never ever saw a user mention guest reboot/performance issues, nor did I
> receive private notes on that.
I am at a loss to explain that. I first started working on putting 
together a few VGA passthrough systems about a year ago. Initially I 
tried with ATI cards with the xm stack and while getting it to work was 
relatively straightforward, it was always problematic when rebooting, 
requiring hacks such as ejecting the card, but in most cases I found 
what happened was that the card would get re-detected and reset before 
the shutdown which meant that it would again be in an unexpected state 
when the VM came back up resulting in either performance degradation, 
complete lack of initialization (VNC VGA only), or the whole host would 
just lock up solid.
> I should note that only recently did I "upgrade" my how-to to use Linux
> Mint 16 / Xen 4.3. I was holding on to Xen 4.1 because later releases
> had the "error 22" bug with xm (and xl probably too), as well as the
> reboot/reset issues discussed here.
I started on 4.1.3, IIRC, and then moved through 4.2 and finally 4.3, 
and I didn't really notice much difference in success rate with ATI VGA 
passthrough.
> So far the only users of this how-to reporting failure to get VGA
> passthrough working with AMD/ATI cards were running Windows 8.1 as domU.
I run XP64, but I did try Windows 7 a few times just to be sure, and 
that had similar issues.
> Of course it's possible that users who managed to get VGA passthrough
> working never reported back on performance/stability issues they may
> have encountered later on, but this seems a little unlikely.
I don't reboot my VMs very often, but having to reboot the whole host 
when I need to reboot any of them is just too much of an inconvenience.
> I'm not questioning your experience, but could there be possibly
> something else involved?
If there is, I don't know what. I had a whole host of other issues that 
I had to work around as well, recently (a couple of months ago) I had a 
HD7970 in one of the VMs again for some testing, and rebooting the VM 
was still just as problematic.
> Essentially what you are saying is that AMD/ATI cards don't work
> reliable in VGA passthrough with Xen xm or xl and don't reset after domU
> shutdown.
That is exactly what I'm saying.

Gordan

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