Per the last messages on this subject from awhile ago, I just ran into this myself. Definitely appears to be an issue on Dell hardware I believe.
Here's how my setup is:
Debian Lenny, 2.6.26-2-amd64, running under Xen 3.2-1 works fine. I then tweaked apt.conf and sources.list so I could access the Xen 3.4 hypervisor package in Squeeze, and installed
_just_ that hypervisor plus its needed dependencies, then manually edited my Grub menu.lst file to reference the new hypervisor kernel.
Reboot, and sure enough, when I see the messages about MSI pop up, they pause for a split second, then the machine reboots automatically. The machine is a Dell M2400 Laptop, Core2 Duo w/ 8GB of RAM. Since the prior
reports also mentioned Dell hardware, this could be something related specifically to Dell's equipment.
The reason I'm trying to switch to the 3.4 hypervisor from Squeeze, is after installing the Windows XP GPLPV drivers in my domU, I discovered the qemu drivers for CD-ROM access no longer functions, which severely
limits my ability to copy updates to my WinXP image. Digging around on some mailing list threads suggested switching to the 3.4 Hypervisor would correct this issue. So if the 3.4 hypervisor can't work, I need to find a way to get CD-ROM access back in XP,
otherwise, I'll have to drop the GPLPV stuff (which is really quite nice).
Thoughts (without using pci=nomsi)? Since this is all from official Debian repos, it should JustWork(), so something isn't clicking here (Probably Dell, but that's the obvious bit).