[RFC 2/3] PowerPC: lazy altivec enabling in kernel

[Paul Mackerras]

Sebastian Siewior writes:

>  void enable_kernel_altivec(void)
>  {
> -	WARN_ON(preemptible());
> +	BUG_ON(preemptible());
> +	/*
> +	 * enable_kernel_altivec() will just save current AltiVec registers (if needed) and
> +	 * return to caller (with MSR_VEC unchanged (probably not set)). The first AltiVec
> +	 * instruction will raise an exception and the exception will enable the AltiVec for
> +	 * the kernel. This is done to avoid the expensive "enable altivec" operation if it
> +	 * is allready enabled. However, you have to disable preemtion while you are using
> +	 * AltiVec.
> +	 */

It would be better to put a test and conditional branch in
giveup_altivec to skip the mtmsrd if MSR_VEC is already set.  That
would avoid adding the overhead of the trap in the case when MSR_VEC
isn't already set, besides being much less code.

Paul.