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

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 03/12/2014 03:34 PM, H. Sieger wrote:
> 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).
I had tried xm to begin with because some reports claimed it was a 
little less problematic for PCI passthrough, but those reports were 
unfounded. The ATI reset bug manifests on both.
> 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?
It doesn't. Have you actually tried it? Every once in a while the domU 
reboot succeeds without crippling the card or crashing the host, but 
more often than not it won't work.
> 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.
AFAIK, it is KVM's addition of bus reset as a means of resetting the 
card that is also being included in Xen. The reason Nvidia cards work is 
because their driver handles the resetting and initialization. ATI 
driver does not. Hypervisor resetting the device to give the driver in 
domU a clean slate is a band-aid for a deficient driver.
Gordan

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