PROBLEM:  (QAR 70150)    (Patch ID: TCR160-004)
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Under certain circumstances a kernel memory fault is experienced on the DRD
client nodes just as or after the DRD server node has initiated MCII hub
failover.

trap: invalid memory read access from kernel mode

    faulting virtual address:     0x2000000000000080
    pc of faulting instruction:   0xfffffc00006cf284
    ra contents at time of fault: 0xfffffc00006da05c
    sp contents at time of fault: 0xffffffffac2db988

panic (cpu 0): kernel memory fault

crash> tf
>  0 thread_block         src/kernel/kern/sched_prim.c : 2352
   1 thread_preempt       src/kernel/kern/sched_prim.c : 4070
   2 boot                 src/kernel/arch/alpha/machdep.c : 1930
   3 panic                src/kernel/bsd/subr_prf.c : 842
   4 trap                 src/kernel/arch/alpha/trap.c : 1747
   5 _XentMM              src/kernel/arch/alpha/locore.s : 1736
   6 drd_map_unref        src/kernel/io/drd/drd_map.c : 2631
   7 bss_rm_biodone       src/kernel/io/drd/bss_rm.c : 1444
   8 bss_rm_iodone_thread  src/kernel/io/drd/bss_rm.c : 1949