On Tue, Nov 13, 2007 at 11:48:53AM +0000, Andrew Haley wrote:
> David Daney writes:
> > With the current kernel (188.8.131.52) 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?
> The best solution is to put the unwinder info in the kernel. Does
> MIPS use a vDSO ?
No though we should.
Another reason is to get rid of the classic trampoline the kernel installs
on the stack. On some multiprocessor systems it requires a cacheflush
operation to be performed on all processors which is expensive. Having
the trampoline in a vDSO would solve that.
I need to look into it, not sure what it would take.