vm: Add kva_alloc_aligned

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

Reviewed by:	alc, kib, markj
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D42788

sys/vm/vm_extern.h

@@ -47,6 +47,7 @@ struct domainset;
 
 /* These operate on kernel virtual addresses only. */
 vm_offset_t kva_alloc(vm_size_t);
+vm_offset_t kva_alloc_aligned(vm_size_t, vm_size_t);
 void kva_free(vm_offset_t, vm_size_t);
 
 /* These operate on pageable virtual addresses. */

sys/vm/vm_kern.c

@@ -155,6 +155,27 @@ kva_alloc(vm_size_t size)
 	return (addr);
 }
 
+/*
+ *	kva_alloc_aligned:
+ *
+ *	Allocate a virtual address range as in kva_alloc where the base
+ *	address is aligned to align.
+ */
+vm_offset_t
+kva_alloc_aligned(vm_size_t size, vm_size_t align)
+{
+	vm_offset_t addr;
+
+	TSENTER();
+	size = round_page(size);
+	if (vmem_xalloc(kernel_arena, size, align, 0, 0, VMEM_ADDR_MIN,
+	    VMEM_ADDR_MAX, M_BESTFIT | M_NOWAIT, &addr))
+		return (0);
+	TSEXIT();
+
+	return (addr);
+}
+
 /*
  *	kva_free:
  *