RFC more sreset debugging improvements

This code seems to never end. This series attempts to make sreset debugging more robust, particularly I'm looking at taking exceptions from CPUs within OPAL. This is starting to work to a degree now (with some skiboot patches I'll post in a minute). At least we can examine registers of the CPU from xmon, can print to console, and sanely crash rather than recover with a trashed OPAL stack.

After this and the skiboot series, we can take a 0x100 and get to xmon like this:

(initramfs) WARNING: cpu 0x0 stopped in OPAL, cannot recover cpu 0x0: Vector: 100 (System Reset) at [c0000000fffcfd80]

    pc: 000000003001b708
    lr: 000000003000515c
    sp: 31c03d20

msr: 9000000002803000
current = 0xc0000000fd862600
paca = 0xc00000000fff0000 softe: 3 irq_happened: 0x01
pid = 16, comm = kopald
Linux version 4.16.0-rc2-00004-g86f2ceed5cac (npiggin@roar) (gcc version 7.3.0 (Debian 7.3.0-1)) #1638 SMP Fri Mar 16 19:53:12 AEST 2018
WARNING: exception is not recoverable, can't continue
enter ? for help
SP (31c03d20) is in userspace
0:mon> x
[ 45.426677142,5] CPU ATTEMPT TO RE-ENTER FIRMWARE! PIR=0000 cpu @0x31c00000 -> pir=0000 token=8

Kernel panic - not syncing: Unrecoverable System Reset
CPU: 0 PID: 16 Comm: kopald Not tainted 4.16.0-rc2-00004-g86f2ceed5cac #1638
Call Trace:

nvram_write_os_partition: Failed nvram_write (-5)

Without the series we end up in a big mess.

Of course it's best not to sreset a CPU that's in OPAL in the first place. I have another few patches for Linux to take a target out of OPAL with quiesce API before sending a sreset. But someitmes a CPU will get stuck in OPAL or we could hit it with pdbg etc.

Thanks,
Nick

Nicholas Piggin (4):
powerpc/64s: return more carefully from sreset NMI powerpc/64s: sreset panic if there is no debugger or crash dump
handlers
powerpc/powernv/nvram: opal_nvram_write handle unknown OPAL errors powerpc/xmon: Detect if OPAL was interrupted and mark unrecoverable

 arch/powerpc/include/asm/opal.h             |  2 +
 arch/powerpc/kernel/exceptions-64s.S        | 61 +++++++++++++++++++++++++++--
 arch/powerpc/kernel/traps.c                 | 15 ++++++-
 arch/powerpc/platforms/powernv/opal-nvram.c |  2 +
 arch/powerpc/platforms/powernv/opal.c       |  5 +++
 arch/powerpc/xmon/xmon.c                    | 14 +++++++
 6 files changed, 94 insertions(+), 5 deletions(-)

2.16.1