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Re: Cannot unwind through MIPS signal frames with ICACHE_REFILLS_WORKARO

To: Franck Bui-Huu <vagabon.xyz@gmail.com>
Subject: Re: Cannot unwind through MIPS signal frames with ICACHE_REFILLS_WORKAROUND_WAR
From: Ralf Baechle <ralf@linux-mips.org>
Date: Tue, 13 Nov 2007 14:00:36 +0000
Cc: Andrew Haley <aph-gcc@littlepinkcloud.com>, David Daney <ddaney@avtrex.com>, linux-mips@linux-mips.org, Richard Sandiford <rsandifo@nildram.co.uk>, gcc@gcc.gnu.org
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On Tue, Nov 13, 2007 at 02:14:58PM +0100, Franck Bui-Huu wrote:

> > > David Daney writes:
> > >  > 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?
> > >
> > > 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.

Oh?

extern void frob(void (*)(void));

int foo(void)
{
        int x;

        void bar(void)
        {
                x++;
        }

        frob(&bar);
        print("x is %d\n", x);
}

Compile and enjoy.

  Ralf

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