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

["Jayachandran C." <jchandra@broadcom.com>]

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.