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Re: Bug in the _save_fp_context.

To: "H . J . Lu" <hjl@lucon.org>
Subject: Re: Bug in the _save_fp_context.
From: Carsten Langgaard <carstenl@mips.com>
Date: Mon, 10 Jun 2002 21:41:42 +0200
Cc: linux-mips@oss.sgi.com
Organization: MIPS Technologies
References: <3C46D2ED.9BCA458A@mips.com> <20020610114323.A25705@lucon.org>
Sender: owner-linux-mips@oss.sgi.com
"H . J . Lu" wrote:

> On Thu, Jan 17, 2002 at 02:34:37PM +0100, Carsten Langgaard wrote:
> > I posted the following problem on the list almost a year ago, but it
> > still hasn't made into the SGI CVS.
> > I think there is a bug in the _save_fp_context function in
> > arch/mips/kernel/r4k_fpu.S
> >
> > The problem is the following piece of code:
> >
> >  jr ra
> >  .set nomacro
> >  EX(sw t0,SC_FPC_EIR(a0))
> >  .set macro
> >
> > We do not handle entries in the __ex_table which is located in a branch
> > delay.
> > So we need to handle the situation where we take a page fault on an
> > instruction which is located in a brach delay slot, or we don't put the
> > "potential" faulting instruction in a delay slot.
> >
> > This situation probably doesn't generally hit people since it hasn't
> > been fix yet, but if you try run something nasty like Crashme, you will
> > get hit by this problem.
> > I suggest the following patch.
> >
> > /Carsten
> >
> >
> > --
> > _    _ ____  ___   Carsten Langgaard   Mailto:carstenl@mips.com
> > |\  /|||___)(___   MIPS Denmark        Direct: +45 4486 5527
> > | \/ |||    ____)  Lautrupvang 4B      Switch: +45 4486 5555
> >   TECHNOLOGIES     2750 Ballerup       Fax...: +45 4486 5556
> >                    Denmark             http://www.mips.com
> >
> >
>
> > Index: arch/mips/kernel/r4k_fpu.S
> > ===================================================================
> > RCS file: /cvs/linux/arch/mips/kernel/r4k_fpu.S,v
> > retrieving revision 1.12
> > diff -u -r1.12 r4k_fpu.S
> > --- arch/mips/kernel/r4k_fpu.S        2001/04/11 05:19:46     1.12
> > +++ arch/mips/kernel/r4k_fpu.S        2002/01/17 14:21:09
> > @@ -50,11 +50,10 @@
> >       EX(sdc1 $f30,(SC_FPREGS+240)(a0))
> >       EX(sw   t1,SC_FPC_CSR(a0))
> >       cfc1    t0,$0                           # implementation/version
> > +     EX(sw   t0,SC_FPC_EIR(a0))
> >
> >       jr      ra
> > -     .set    nomacro
> > -      EX(sw  t0,SC_FPC_EIR(a0))
> > -     .set    macro
> > +      nop
> >       END(_save_fp_context)
> >
> >  /*
>
> Do we still need this patch for 2.4 on OSS?
>
> H.J.

No, this fix is not needed any more, as we now handle entries in the
__ex_table which is located in a branch delay.



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