Re: Xen 4.3 Release VGA Passthrough Questions

[George Dunlap <George.Dunlap@eu.citrix.com>]

On Fri, Jul 19, 2013 at 11:55 PM, Casey DeLorme <cdelorme@gmail.com> wrote:
> Hello xen-devel,
>
> I was hoping to ask some VGA passthrough questions with regards to the 4.3
> release, and going forward.  I love Xen, and want to help iron out the
> problems in relation to passthrough.  I am very impressed with the
> performance of upstream qemu, especially network management.
>
>
> I use Debian as my Dom0, as it is the linux distro I am most familiar with.
> My key hardware includes an Intel Core i7 3770 IvyBridge CPU, ASRock Z77
> Extreme9 Motherboard, and an AMD Radeon HD 6870 GPU.  My system has 32GB of
> RAM and I have been running an average of 4 virtual machines at a time when
> I was using Xen 4.2.
>
>
> The problems I would like to address area:
>
> - RAM Limitations /w Upstream Qemu
> - Performance Degredation
> - Primary Passthrough
>
>
> Upstream Qemu and VGA Passthrough limits my supplied RAM.  In my case
> supplying anything more than 3584MB and the machine boots but the graphics
> card is not used (but does exist).  I tried applying a patch that was sent
> around the xen-users list, but with that patch my Windows HVM (even without
> any changes to RAM) goes to an infinite boot screen in windows (as seen
> through sdl or vnc).

What guest are you using?

There is a known issue with qemu-upstream and PCI pass-through that we
found too late in the release cycle to do a proper fix, so we had to
do a work-around.  Behavior like what you describe was known to be one
of the possible side-effects of the work-around, particularly for
32-bit guests, but it's still better than having the guest crash
(which is what would happen without the work-around).

Re your other issues, it's better if you send one issue per e-mail.
They also need more detail to be useful -- see below.

> While I am able to overcome these limitations by switching to qemu
> traditional, I have other problems that occassionally kill my network when I
> hit heavy traffic.  These may be related to GPLPV and not Xen, but
> attempting to re-enable the adpater fails, the adapter disappears from the
> system, and my only option is to reboot.  Disabling the adapter with
> upstream and GPLPV has the same issue, but I have not encountered the same
> problem with the network adapter crashing under load.

I've heard this issue before -- CC the GPLPV maintainer when you send your mail.

> The performance degradation problem exists in both upstream and traditional,
> and may have nothing to do with Xen.  I don't expect the devels to solve
> this, but it would be nice if they could share some knowledge since my
> understanding of linux is likely vastly inferior.

Well you haven't said exactly what you're seeing, so short of doing a
full brain dump, it's not clear what particular knowledge to share
with you. :-)  You need to describe in more detail your setup, and how
exactly you're measuring the performance.

 -George