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

["H. Sieger" <powerhouse.linux@xxxxxxxxx>]

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 and HOW-TO make dual-boot obsolete using XEN VGA passthrough, or http://www.overclock.net/t/1205216/guide-create-a-gaming-virtual-machine. Since I wrote the "how-to make dual-boot obsolete..." 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 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.

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. 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'm not questioning your experience, but could there be possibly something else involved?

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.

On Wednesday, March 12, 2014 7:01 PM, Gordan Bobic <gordan@xxxxxxxxxx> wrote:

On 03/12/2014 03:42 PM, H. Sieger wrote:
> Thanks for the reply! Good to know the old ATI patch still works with
> Xen 4.4.

If you are talking about the patch I think you are talking about, IIRC
that was a patch for primary passthrough. This side-steps some of the
problems of lack of reset by POST-ing the VBIOS in domU, so VBIOS gets
the card into a state where the driver knows what to do with it.

> Unfortunately I use a Nvidia Quadro card for VGA passthrough and can't
> test it easily. However, some users on the Xen VGA passthrough how-to I
> posted on the Linux Mint forum have reported problems. I usually advise
> to use the xm toolstack as that doesn't run into issues with ATI
> graphics cards, but it doesn't work with Windows 8.1 guests.

I'm still not sure where you got the idea that xm stack works any better
when it comes to ATI passthrough and resetting. Neither tool stack in
4.3.x resets the ATI cards, and this results in the card being passed to
domU the second time around in an unexpected state which makes the
driver fall over.

> I really wish someone would integrate the ATI patch into a Xen release
> and be done with it. Though I've never tried it, I've seen people apply
> the patch and it seems to solve the problem.

I'm pretty sure there are two different patches we are talking about
here. One enables primary passthrough with VBIOS POST-ing in domU, the
other adds PCI bus reset as a means of resetting the device before
passing it through.

Gordan

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