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Re: [Xen-users] Gettind ready for a computer build for IOMMU virtualization, need some input regarding sharing the GPU among VMs

Running a slim hypervisor (based on Arch linux) and Linux as well as Windows
in VMs may be a text book approach, but make things more complicated than
they are already for someone with little Linux experience. Arch Linux itself
may be a little more challenging than say a desktop distribution such as
Linux Mint. While Arch is more up-to-date (or bleeding edge) than say Ubuntu
or Debian, you may have to do some tweaking. However, the Arch Linux
documentation is the best you can find.

I would go a much simpler way, unless you really have a need for making
things more complicated (multiple VMs, thin hypervisor, etc.). What I did is
install a user friendly desktop Linux OS (Linux Mint 14 Mate, to be
precise), then the Xen hypervisor from repository and finally the Windows 7
64 bit VM with VGA passthrough. Here is my how-to, if you are interested: 
</a> <http://forums.linuxmint.com/viewtopic.php?f=42&t=112013> .

I also wrote some little backup scripts and other useful utility scripts and
everything works smooth for well over a year now. The only hickup I had was
a Xen update that introduced a nasty error-22 bug (if I remember correctly
the Xen devs have fixed it, but Ubuntu/Linux Mint are a bit behind in
releasing it).

Using a full fledged desktop OS as dom0 (the administrative domain) has its
pros and cons, but for private users I believe the pros outweigh the cons.
So my rig has only 2 GPUs - one for dom0/Linux Mint and one for my Windows
VM. Both cards are connected to one screen - the screen has 2 DVI inputs and
a VGA input.

In your case I would do as follows (if you accept my suggestion of dumping
the slim hypervisor):

1. Intel GPU (inside CPU) for Linux / dom0 - set in BIOS as the main GPU to
boot with - connected to screen A
2. 1st graphics card for Windows VM (XP or what you use mainly) - connected
to screen B
3. 2nd graphics card for Windows 7 or a Linux guest, used only when needed -
connected to screen A

Not sure about your screens but mine and many I have seen allow switching
between multiple inputs (if they have).

The above configuration also makes it easy to blacklist the AMD drivers, as
you won't need them for your Linux dom0.

As far as I know, you can't just bind or unbind graphics drivers at will.
Once you booted your Windows VM with VGA passthrough, that graphics card is
now claimed by Windows. Even after closing the Windows VM chances are that
you won't be able to use that graphics card for say a Linux guest (with VGA
passthrough). I haven't tried newer Xen releases nor the xl tool stack, so
things may be a little different now. But I would not build on that. On the
other hand, it's very easy to reclaim your USB devices for dom0 and thus for
other VMs.

My advice is to take it easy at the beginning and start with a basic
configuration with VGA passthrough (secondary passthrough). There are enough
things that can go wrong there.

Regarding the 2GB limit for domUs (VMs), I never heard about that. My
Windows 7 domU gets 24GB.

By the way, 32GB are plenty for gaming rigs. The only time I've seen real
demand for memory is doing photo or video processing, or audio
recording/processing. In Linux you can easily assign the /tmp folder(s) to
RAM, and fine tune access to a swap partition (if you need one - I prefer to
have it).

Good luck.

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