arm: Allow in-kernel VFP instructions handling.

[andrew]

Why is the kernel getting into the undefined instruction handler? It should only use VFP instructions from within a specific section marked by the fpu_kern_* functions

[markj]

In D41759#951401, @andrew wrote:

Why is the kernel getting into the undefined instruction handler? It should only use VFP instructions from within a specific section marked by the fpu_kern_* functions

As I understand it, your comment is only true when fpu_kern_enter() is called with FPU_KERN_NOCTX. When an explicit context is provided, I believe the intent is to enable VFP lazily, but that is currently broken. The intent of the patch is to fix fpu_kern_enter() to behave properly when the caller supplies a context.

sys/arm/arm/vfp.c

281

This addition is unrelated to the rest of the patch and should be added separately.

[des]

sys/arm/arm/undefined.c

354