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Re: [PATCH 2/5] MIPS: Allow kernel to use coprocessor 2

To: "Ralf Baechle" <ralf@linux-mips.org>
Subject: Re: [PATCH 2/5] MIPS: Allow kernel to use coprocessor 2
From: "Jayachandran C." <jchandra@broadcom.com>
Date: Tue, 11 Jun 2013 09:42:54 +0530
Cc: linux-mips@linux-mips.org, ddaney.cavm@gmail.com
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References: <1370849404-4918-1-git-send-email-jchandra@broadcom.com> <1370849404-4918-3-git-send-email-jchandra@broadcom.com> <20130610124229.GH28380@linux-mips.org>
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On Mon, Jun 10, 2013 at 02:42:29PM +0200, Ralf Baechle wrote:
> On Mon, Jun 10, 2013 at 01:00:01PM +0530, Jayachandran C wrote:
> 
> > Kernel threads should be able to use COP2 if the platform needs it.
> > Do not call die_if_kernel() for a coprocessor unusable exception if
> > the exception due to COP2 usage.  Instead, the default notifier for
> > COP2 exceptions is updated to call die_if_kernel.
> > 
> > Signed-off-by: Jayachandran C <jchandra@broadcom.com>
> > ---
> >  arch/mips/kernel/traps.c |   15 +++++----------
> >  1 file changed, 5 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
> > index beba1e6..142d2be 100644
> > --- a/arch/mips/kernel/traps.c
> > +++ b/arch/mips/kernel/traps.c
> > @@ -1056,15 +1056,9 @@ static int default_cu2_call(struct notifier_block 
> > *nfb, unsigned long action,
> >  {
> >     struct pt_regs *regs = data;
> >  
> > -   switch (action) {
> > -   default:
> > -           die_if_kernel("Unhandled kernel unaligned access or invalid "
> > +   die_if_kernel("COP2: Unhandled kernel unaligned access or invalid "
> >                           "instruction", regs);
> > -           /* Fall through  */
> > -
> > -   case CU2_EXCEPTION:
> > -           force_sig(SIGILL, current);
> > -   }
> > +   force_sig(SIGILL, current);
> >  
> >     return NOTIFY_OK;
> >  }
> 
> The idea for cu2_chain notifiers is that they should return a value with
> NOTIFY_STOP_MASK set.  That would prevent further calls to other notifiers.
> The default_cu2_call() notifier is installed with the lowest possible
> priority and is meant to only be called if there is no other notifier was
> installed, that is on a bog standard MIPS core with no CP2.

I am trying to move the die_if_kernel from do_cpu() to here for COP2
exceptions. In the existing code, COP2 unusable exception calls
die_if_kernel in do_cpu() itself, so COP2 unusable generated by kernel
threads will not make it to the notifiers.  With this change (and the
one below), die_if_kernel() is called only if the all the notifiers
pass it thru.

> 
> > @@ -1080,10 +1074,11 @@ asmlinkage void do_cpu(struct pt_regs *regs)
> >     unsigned long __maybe_unused flags;
> >  
> >     prev_state = exception_enter();
> > -   die_if_kernel("do_cpu invoked from kernel context!", regs);
> > -
> >     cpid = (regs->cp0_cause >> CAUSEB_CE) & 3;
> >  
> > +   if (cpid != 2)
> > +           die_if_kernel("do_cpu invoked from kernel context!", regs);
> > +
> >     switch (cpid) {
> >     case 0:
> >             epc = (unsigned int __user *)exception_epc(regs);
> 
> I'm ok with this segment.
> 
>   Ralf

JC.


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