[PATCH v3 4/9] powerpc: Explicitly disable math features when copying thread

[Balbir Singh]

On Thu, 21 Jan 2016 11:55:44 +1100
Cyril Bur <cyrilbur at gmail.com> wrote:

> Currently when threads get scheduled off they always giveup the FPU,
> Altivec (VMX) and Vector (VSX) units if they were using them. When they are
> scheduled back on a fault is then taken to enable each facility and load
> registers. As a result explicitly disabling FPU/VMX/VSX has not been
> necessary.
> 
> Future changes and optimisations remove this mandatory giveup and fault
> which could cause calls such as clone() and fork() to copy threads and run
> them later with FPU/VMX/VSX enabled but no registers loaded.
> 
> This patch starts the process of having MSR_{FP,VEC,VSX} mean that a
> threads registers are hot while not having MSR_{FP,VEC,VSX} means that the
> registers must be loaded. This allows for a smarter return to userspace.
> 
> Signed-off-by: Cyril Bur <cyrilbur at 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 dccc87e..e0c3d2d 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1307,6 +1307,7 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
>  
>  		f = ret_from_fork;
>  	}
> +	childregs->msr &= ~(MSR_FP|MSR_VEC|MSR_VSX);

Ideally you want to use __msr_check_and_clear()

Basically we start with these bits off and then take an exception on use?

>  	sp -= STACK_FRAME_OVERHEAD;
>  
>  	/*