The bad stack test in interrupt handlers has a few problems. For
performance it is taken in the common case, which is a fetch bubble
and a waste of i-cache.

For code development and maintainence, it requires yet another stack
frame setup routine, and that constrains all exception handlers to
follow the same register save pattern which inhibits future
optimisation.

Remove the test/branch and replace it with a trap. Teach the program
check handler to use the emergency stack for this case.

This does not result in quite so nice a message, however the SRR0 and
SRR1 of the crashed interrupt can be seen in r11 and r12, as is the
original r1 (adjusted by INT_FRAME_SIZE). These are the most important
parts to debugging the issue.

The original r9-12 and cr0 is lost, which is the main downside.

  kernel BUG at linux/arch/powerpc/kernel/exceptions-64s.S:847!
  Oops: Exception in kernel mode, sig: 5 [#1]
  BE SMP NR_CPUS=2048 NUMA PowerNV
  Modules linked in:
  CPU: 0 PID: 1 Comm: swapper/0 Not tainted
  NIP:  c000000000009108 LR: c000000000cadbcc CTR: c0000000000090f0
  REGS: c0000000fffcbd70 TRAP: 0700   Not tainted
  MSR:  9000000000021032 <SF,HV,ME,IR,DR,RI>  CR: 28222448  XER: 20040000
  CFAR: c000000000009100 IRQMASK: 0
  GPR00: 000000000000003d fffffffffffffd00 c0000000018cfb00 c0000000f02b3166
  GPR04: fffffffffffffffd 0000000000000007 fffffffffffffffb 0000000000000030
  GPR08: 0000000000000037 0000000028222448 0000000000000000 c000000000ca8de0
  GPR12: 9000000002009032 c000000001ae0000 c000000000010a00 0000000000000000
  GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  GPR20: c0000000f00322c0 c000000000f85200 0000000000000004 ffffffffffffffff
  GPR24: fffffffffffffffe 0000000000000000 0000000000000000 000000000000000a
  GPR28: 0000000000000000 0000000000000000 c0000000f02b391c c0000000f02b3167
  NIP [c000000000009108] decrementer_common+0x18/0x160
  LR [c000000000cadbcc] .vsnprintf+0x3ec/0x4f0
  Call Trace:
  Instruction dump:
  996d098a 994d098b 38610070 480246ed 48005518 60000000 38200000 718a4000
  7c2a0b78 3821fd00 41c20008 e82d0970 <0981fd00> f92101a0 f9610170 f9810178
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>