alloc_vaddr
va = (caddr_t)alloc_vaddr(len, MMU_PAGESIZE);
virthint = (caddr_t)alloc_vaddr(size, a);
extern uintptr_t alloc_vaddr(size_t, paddr_t);
(caddr_t)alloc_vaddr(MMU_PAGESIZE, MMU_PAGESIZE);