On Nov 13, 2007 1:10 PM, Ralf Baechle <firstname.lastname@example.org> wrote:
> On Tue, Nov 13, 2007 at 11:48:53AM +0000, Andrew Haley wrote:
> > David Daney writes:
> > > With the current kernel (220.127.116.11) 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.
And the stack wouldn't need to have exec permission anymore.
> I need to look into it, not sure what it would take.
I started to add vdso support for MIPS a couple months ago, but
it's in a very early stage and I unfortunately haven't time to finish
it. I can send it to you if you want.