Meelis Roos reported that kernels built with gcc-4.9 do not boot, we
eventually narrowed this down to only impacting machines using
UltraSPARC-III and derivitive cpus.
The crash happens right when the first user process is spawned:
[ 54.451346] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004
[ 54.451346]
[ 54.571516] CPU: 1 PID: 1 Comm: init Not tainted 3.16.0-rc2-00211-gd7933ab #96
[ 54.666431] Call Trace:
[ 54.698453] [0000000000762f8c] panic+0xb0/0x224
[ 54.759071] [000000000045cf68] do_exit+0x948/0x960
[ 54.823123] [000000000042cbc0] fault_in_user_windows+0xe0/0x100
[ 54.902036] [0000000000404ad0] __handle_user_windows+0x0/0x10
[ 54.978662] Press Stop-A (L1-A) to return to the boot prom
[ 55.050713] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004
Further investigation showed that compiling only per_cpu_patch() with
an older compiler fixes the boot.
Detailed analysis showed that the function is not being miscompiled by
gcc-4.9, but it is using a different register allocation ordering.
With the gcc-4.9 compiled function, something during the code patching
causes some of the %i* input registers to get corrupted. Perhaps
we have a TLB miss path into the firmware that is deep enough to
cause a register window spill and subsequent restore when we get
back from the TLB miss trap.
Let's plug this up by doing two things:
1) Stop using the firmware stack for client interface calls into
the firmware. Just use the kernel's stack.
2) As soon as we can, call into a new function "start_early_boot()"
to put a one-register-window buffer between the firmware's
deepest stack frame and the top-most initial kernel one.
Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: David S. Miller <davem@davemloft.net>
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