[PATCH v5 0/3] Refactor FTLBPar exception handling and add GSExc handler

[WANG Xuerui <git@xen0n.name>]

It was found that some undocumented unprivileged instructions could
crash the kernel with a "FTLB parity error" on Loongson-3A4000, but the
error registers were garbage:

[  896.970419] FTLB error exception, cp0_ecc=0x00000002:
[  896.975446] cp0_errorepc == ffffffffffffffff
[  896.979755] c0_cacheerr == 00000000
[  896.983277] Decoded c0_cacheerr: primary cache fault in insn reference.
[  896.989963] Cache error exception:
[  896.993396] cp0_errorepc == ffffffffffffffff
[  896.997707] c0_cacheerr == 00000000
[  897.001228] Decoded c0_cacheerr: primary cache fault in insn reference.
[  897.007916] Error bits: 
[  897.010467] IDX: 0x00000000
[  897.013284] Kernel panic - not syncing: Can't handle the cache error!
[  897.019807] ---[ end Kernel panic - not syncing: Can't handle the cache error! ]---

Turns out the FTLBPar exception code is actually implementation-specific.
On Loongson cores the exception is "GSExc" instead, and is non-fatal in
Loongson's linux-3.10 fork. So we dynamically register the correct handler
for the exception, and do not panic on the specific undocumented case.

P.S. There is not much space left in the cpuinfo_mips.options flag. We
should consider moving to something like x86's feature flags that is
extensible.

v5:
- Fixed broken build on pre-MIPS32 configs where mfc0 can't have
  selector, pointed out by Thomas. Build-tested ip22_defconfig.
- Added more description in the commit message of the 2nd patch

v4:
- Added Reviewed-by tags from Huacai
- Dropped bouncing Cc addresses from the first patch
- Fixed "MFC0" -> "mfc0" in genex.S, as that CP0 register is 32-bit
  according to Loongson's manuals

v3:
- Simplified declaration of cpu_has_gsexcex, as suggested by Huacai

v2:
- Removed stray GSExc logic in cpu_probe_legacy, pointed out by Huacai

WANG Xuerui (3):
  MIPS: only register FTLBPar exception handler for supported models
  MIPS: add definitions for Loongson-specific CP0.Diag1 register
  MIPS: handle Loongson-specific GSExc exception

 arch/mips/include/asm/cpu-features.h |  8 ++++++
 arch/mips/include/asm/cpu.h          |  2 ++
 arch/mips/include/asm/mipsregs.h     | 11 ++++++++
 arch/mips/kernel/cpu-probe.c         | 16 ++++++++++++
 arch/mips/kernel/genex.S             | 14 ++++++++++
 arch/mips/kernel/traps.c             | 38 +++++++++++++++++++++++++++-
 6 files changed, 88 insertions(+), 1 deletion(-)

-- 
2.25.1