diff --git a/sys/arm64/include/vmm.h b/sys/arm64/include/vmm.h
--- a/sys/arm64/include/vmm.h
+++ b/sys/arm64/include/vmm.h
@@ -102,14 +102,30 @@
 #define	VM_INTINFO_HWEXCEPTION	(3 << 8)
 #define	VM_INTINFO_SWINTR	(4 << 8)
 
-#define VM_MAX_SUFFIXLEN 15
-
 #define VM_GUEST_BASE_IPA	0x80000000UL	/* Guest kernel start ipa */
 
-#ifdef _KERNEL
-
-#define	VM_MAX_NAMELEN	32
+/*
+ * The VM name has to fit into the pathname length constraints of devfs,
+ * governed primarily by SPECNAMELEN.  The length is the total number of
+ * characters in the full path, relative to the mount point and not 
+ * including any leading '/' characters.
+ * A prefix and a suffix are added to the name specified by the user.
+ * The prefix is usually "vmm/" or "vmm.io/", but can be a few characters
+ * longer for future use.
+ * The suffix is a string that identifies a bootrom image or some similar
+ * image that is attached to the VM. A separator character gets added to
+ * the suffix automatically when generating the full path, so it must be
+ * accounted for, reducing the effective length by 1.
+ * The effective length of a VM name is 229 bytes for FreeBSD 13 and 37
+ * bytes for FreeBSD 12.  A minimum length is set for safety and supports
+ * a SPECNAMELEN as small as 32 on old systems.
+ */
+#define VM_MAX_PREFIXLEN 10
+#define VM_MAX_SUFFIXLEN 15
+#define VM_MAX_NAMELEN \
+    (SPECNAMELEN - VM_MAX_PREFIXLEN - VM_MAX_SUFFIXLEN - 1)
 
+#ifdef _KERNEL
 struct vm;
 struct vm_exception;
 struct vm_exit;