Cannot unwind through MIPS signal frames with ICACHE_REFILLS_WORKAROUND_WAR

[David Daney <ddaney@avtrex.com>]

With the current kernel (2.6.23.1) in my R5000 based O2 it seems 
impossible for GCC's exception unwinding machinery to unwind through 
signal frames.  The cause of the problems is the 
ICACHE_REFILLS_WORKAROUND_WAR which puts the sigcontext at an almost 
impossible to determine offset from the signal return trampoline.  The 
unwinder depends on being able to find the sigcontext given a known 
location of the trampoline.

It seems there are a couple of possible solutions:

1) The comments in war.h indicate the problem only exists in R7000 and 
E9000 processors.  We could turn off the workaround if the kernel is 
configured for R5000.  That would help me, but not those with the 
effected systems.

2) In the non-workaround case, the siginfo immediately follows the 
trampoline and the first member is the signal number.  For the 
workaround case the first word following the trampoline is zero.  We 
could replace this with the offset to the sigcontext which is always a 
small negative value.  The unwinder could then distinguish the two cases 
(signal numbers are positive and the offset negative).  If we did this, 
the change would have to be coordinated with GCC's unwinder (in 
libgcc_s.so.1).

Thoughts?

David Daney