looking at entry 0x0400 of the interrupt table I am wondering about the
add r21=16,r18 // tag
add r20=24,r18 // collision chain
ld8 r21=[r21] // check VHPT tag
Are you sure you cannot enter a nested TLB miss fault here on IA64
implementations that do not have the VHPT walker implemented? You
wouldn't get fault 0x0000 since there is no walker that throws that fault.