AMD has released a range of quad-core, 8-core, 12-core and 16-core processors featuring its second-generation 'Piledriver' modular core architecture which are designed as a drop-in replacement for 'Bulldozer' Opteron parts.
I experienced core dumps (Illegal Instruction) on 2 new Opteron 2-way 6274 systems while using XEN 4.1.2.
I could make following observations in different configurations:
2x Opteron 6274: Ubuntu Precise w/o XEN = WORKING
2x Opteron 6274: Ubuntu Precise + XEN 4.1.2 = NOT working
2x Opteron 6274: Ubuntu Oneiric + XEN 4.1.1 = WORKING
2x Opteron 6274: Fedora 16 + XEN 4.1.2 + Kernel 3.1 = WORK