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Re: Cannot unwind through MIPS signal frames with ICACHE_REFILLS_WORKARO

To: "Ralf Baechle" <ralf@linux-mips.org>
Subject: Re: Cannot unwind through MIPS signal frames with ICACHE_REFILLS_WORKAROUND_WAR
From: "Franck Bui-Huu" <vagabon.xyz@gmail.com>
Date: Tue, 13 Nov 2007 14:14:58 +0100
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 Nov 13, 2007 1:10 PM, Ralf Baechle <ralf@linux-mips.org> wrote:
>
> On Tue, Nov 13, 2007 at 11:48:53AM +0000, Andrew Haley 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.

> 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.

-- 
               Franck

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