Re: [PATCH 4/8] powerpc: Explicitly disable math features when copying thread

[Cyril Bur <cyrilbur@gmail.com>]

On Mon, 23 Nov 2015 12:08:38 +1100
Michael Neuling <mikey@neuling.org> wrote:

> On Wed, 2015-11-18 at 14:26 +1100, Cyril Bur wrote:
> > With threads leaving the math bits enabled in their saved MSR to
> > indicate
> > that the hardware is hot and a restore is not needed, children need
> > to turn
> > it off as when they do get scheduled, there's no way their registers
> > could
> > have been hot.
> > 
> > Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
> > ---
> >  arch/powerpc/kernel/process.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/powerpc/kernel/process.c
> > b/arch/powerpc/kernel/process.c
> > index 398f7bf..441d9e5 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -1243,6 +1243,7 @@ int copy_thread(unsigned long clone_flags,
> > unsigned long usp,
> >  
> >  		f = ret_from_fork;
> >  	}
> > +	childregs->msr &= ~(MSR_FP|MSR_VEC|MSR_VSX);  
> 
> Is this a current bug? 
> 

It is impossible currently because saving the registers (of the parent, before
the creating the child) also forces a giveup of the facilities.

The next patch in the series decouples the saving and the giving up which
makes this situation possible.

> Mikey
> 
> >  	sp -= STACK_FRAME_OVERHEAD;
> >  
> >  	/*