vm: Add kva_alloc_aligned

[andrew]

Summary

Add a function like kva_alloc that allows us to specify the alignment
of the virtual address space returned.

Sponsored by: Arm Ltd

[markj]

sys/vm/vm_kern.c

171

See my comment in D42787 - I think this can simply use vmem_xalloc() when align > PAGE_SIZE. Using vmem_xalloc() unconditionally would also be fine.

[alc]

sys/vm/vm_kern.c

170

This is unnecessary both here and in kva_alloc() and kva_free() because the kernel_arena is initialized with PAGE_SIZE as the allocation granularity.

171

Agreed. The only catch is that the documentation for vmem says that allocations obtained from vmem_xalloc() must be freed with vmem_xfree() (presumably so that freed allocations don't wind up in the quantum cache where vmem_xalloc() couldn't reallocate them). We don't enable the quantum cache for the kernel_arena, and I don't see us changing that, so we can simply change kva_alloc() and kva_free() to use vmem_xalloc() and vmem_xfree(), respectively.

[andrew]

Use vmem_xalloc

[andrew]

sys/vm/vm_kern.c

171

The alignment should probably be at least PAGE_SIZE as the smallest allocation is a page. I don't have a check, but could add one.

[alc]

sys/vm/vm_kern.c

171

It would be redundant. At initialization, we specify the page size as the granularity of allocation for this arena. Any such allocation will satisfy non-zero alignment values that are less than the page size.