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

To: Jayachandran C <jchandra@broadcom.com>
Subject: Re: [PATCH 2/5] MIPS: Allow kernel to use coprocessor 2
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 10 Jun 2013 14:42:29 +0200
Cc: linux-mips@linux-mips.org, ddaney.cavm@gmail.com
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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.

> @@ -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

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