[libvirt-users] Why a max mem setting?

Hi I have just started to investigate the possibilities of resizing memory for my kvm virtual machines on the fly. It works just fine with virsh setmem and that's great. Now, what I'm wondering about is the <memory> directive in the xml domain definition. Why would I not just set that to the size of available RAM in the host machine? Am I missing some nuance or finesse to the handling of memory? What would happen if I had three virtual machines with max set to physical max and then used setmem to give one of the virtual machines more than its third? Thank for any feedback. I could maybe just try the latter out, but I have no machine I could crash if that's the result... /andreas