RE: getcpu() returns EFAULT when called via the vdso

["Luck, Tony" <tony.luck@xxxxxxxxx>]

the disassembly of the sched_getcpu() code at runtime looks like:
   0x2000000000203940 <+0>:     [MMI]       alloc r32=ar.pfs,9,1,0
   0x2000000000203941 <+1>:                 adds r14=8,r13
   0x2000000000203942 <+2>:                 mov r33=r12;;
   0x2000000000203950 <+16>:    [MMI]       ld8 r14=[r14]
   0x2000000000203951 <+17>:                nop.m 0x0
   0x2000000000203952 <+18>:                mov r15=1304
   0x2000000000203960 <+32>:    [MII]       mov r35=r0  <<<<<<<<<<<<<<<<<<<
   0x2000000000203961 <+33>:                mov r34=r0;;<<<<<<<<<<<<<<<<<<<
   0x2000000000203962 <+34>:                mov b7=r14;;
   0x2000000000203970 <+48>:    [MIB]       nop.m 0x0
   0x2000000000203971 <+49>:                nop.i 0x0
   0x2000000000203972 <+50>:                br.call.sptk.many b6=b7;;

When the "br.call" is executed, we flip to the new register
frame and r34/r35 in the sched_getcpu() frame become r32/r33
in the new frame.

So you get -EFAULT because the VDSO tries to dereference a NULL
pointer for each of the *cpu and *node arguments.

-Tony
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