As I have described in a previous patch, the Xen hypervisor has a very nasty
bug that causes all of the x2apic msr writes to trigger a GPF. Although the
request proceeds fine despite the GPF, it does bring a problem for all-but-self
style init sequences we are using: after "failing" (succeeding but returning
failure) to deliver the interrupt for the first cpu in the group, xen will
break the loop, therefore not delivering the SIPIs to other cpus in the system
at all. We can work around that by delivering interrupts to each cpu
individually, instead of all-but-self.