diff --git a/usr.sbin/bhyve/aarch64/Makefile.inc b/usr.sbin/bhyve/aarch64/Makefile.inc
--- a/usr.sbin/bhyve/aarch64/Makefile.inc
+++ b/usr.sbin/bhyve/aarch64/Makefile.inc
@@ -1,5 +1,6 @@
 SRCS+=	\
 	fdt.c		\
+	mem_md.c	\
 	rtc_pl031.c	\
 	uart_pl011.c
 
diff --git a/usr.sbin/bhyve/amd64/Makefile.inc b/usr.sbin/bhyve/amd64/Makefile.inc
--- a/usr.sbin/bhyve/amd64/Makefile.inc
+++ b/usr.sbin/bhyve/amd64/Makefile.inc
@@ -7,6 +7,7 @@
 	inout.c		\
 	ioapic.c	\
 	kernemu_dev.c	\
+	mem_md.c	\
 	mptbl.c		\
 	pci_fbuf.c	\
 	pci_gvt-d.c	\
diff --git a/usr.sbin/bhyve/mem.h b/usr.sbin/bhyve/mem.h
--- a/usr.sbin/bhyve/mem.h
+++ b/usr.sbin/bhyve/mem.h
@@ -53,6 +53,8 @@
 void	init_mem(int ncpu);
 int     emulate_mem(struct vcpu *vcpu, uint64_t paddr, struct vie *vie,
 		    struct vm_guest_paging *paging);
+int	mmio_handle_non_backed_mem(struct vcpu *vcpu __unused, uint64_t paddr,
+		    struct mem_range **mr_paramp);
 
 int	read_mem(struct vcpu *vpu, uint64_t gpa, uint64_t *rval, int size);
 int	register_mem(struct mem_range *memp);
diff --git a/usr.sbin/bhyve/mem.c b/usr.sbin/bhyve/mem.c
--- a/usr.sbin/bhyve/mem.c
+++ b/usr.sbin/bhyve/mem.c
@@ -33,6 +33,7 @@
  */
 
 #include <sys/types.h>
+#define _WANT_KERNEL_ERRNO 1
 #include <sys/errno.h>
 #include <sys/tree.h>
 #include <machine/vmm.h>
@@ -167,10 +168,13 @@
 access_memory(struct vcpu *vcpu, uint64_t paddr, mem_cb_t *cb, void *arg)
 {
 	struct mmio_rb_range *entry;
+	struct mem_range *mr;
 	int err, perror, immutable, vcpuid;
 
 	vcpuid = vcpu_id(vcpu);
+	mr = NULL;
 	pthread_rwlock_rdlock(&mmio_rwlock);
+
 	/*
 	 * First check the per-vCPU cache
 	 */
@@ -185,14 +189,22 @@
 		if (mmio_rb_lookup(&mmio_rb_root, paddr, &entry) == 0) {
 			/* Update the per-vCPU cache */
 			mmio_hint[vcpuid] = entry;
-		} else if (mmio_rb_lookup(&mmio_rb_fallback, paddr, &entry)) {
-			perror = pthread_rwlock_unlock(&mmio_rwlock);
-			assert(perror == 0);
-			return (ESRCH);
+		} else if (mmio_rb_lookup(&mmio_rb_fallback, paddr,
+		    &entry) == 0) {
+		} else {
+			err = mmio_handle_non_backed_mem(vcpu, paddr, &mr);
+			if (err != 0) {
+				perror = pthread_rwlock_unlock(&mmio_rwlock);
+				assert(perror == 0);
+				return (err == EJUSTRETURN ? 0 : err);
+			}
 		}
 	}
 
-	assert(entry != NULL);
+	if (mr == NULL) {
+		assert(entry != NULL);
+		mr = &entry->mr_param;
+	}
 
 	/*
 	 * An 'immutable' memory range is guaranteed to be never removed
@@ -205,13 +217,13 @@
 	 * deadlock on 'mmio_rwlock'. However by registering the extended
 	 * config space window as 'immutable' the deadlock can be avoided.
 	 */
-	immutable = (entry->mr_param.flags & MEM_F_IMMUTABLE);
+	immutable = (mr->flags & MEM_F_IMMUTABLE) != 0;
 	if (immutable) {
 		perror = pthread_rwlock_unlock(&mmio_rwlock);
 		assert(perror == 0);
 	}
 
-	err = cb(vcpu, paddr, &entry->mr_param, arg);
+	err = cb(vcpu, paddr, mr, arg);
 
 	if (!immutable) {
 		perror = pthread_rwlock_unlock(&mmio_rwlock);
diff --git a/usr.sbin/bhyve/mem.h b/usr.sbin/bhyve/mem_md.c
copy from usr.sbin/bhyve/mem.h
copy to usr.sbin/bhyve/mem_md.c
--- a/usr.sbin/bhyve/mem.h
+++ b/usr.sbin/bhyve/mem_md.c
@@ -1,8 +1,10 @@
 /*-
  * SPDX-License-Identifier: BSD-2-Clause
  *
- * Copyright (c) 2012 NetApp, Inc.
- * All rights reserved.
+ * Copyright (c) 2025 The FreeBSD Foundation
+ *
+ * This software was developed by Konstantin Belousov <kib@FreeBSD.org>
+ * under sponsorship from the FreeBSD Foundation.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -13,10 +15,10 @@
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
- * THIS SOFTWARE IS PROVIDED BY NETAPP, INC ``AS IS'' AND
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL NETAPP, INC OR CONTRIBUTORS BE LIABLE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
@@ -26,38 +28,17 @@
  * SUCH DAMAGE.
  */
 
-#ifndef _MEM_H_
-#define	_MEM_H_
-
-#include <sys/linker_set.h>
-
-struct vcpu;
-
-typedef int (*mem_func_t)(struct vcpu *vcpu, int dir, uint64_t addr,
-			  int size, uint64_t *val, void *arg1, long arg2);
-
-struct mem_range {
-	const char 	*name;
-	int		flags;
-	mem_func_t	handler;
-	void		*arg1;
-	long		arg2;
-	uint64_t  	base;
-	uint64_t  	size;
-};
-#define	MEM_F_READ		0x1
-#define	MEM_F_WRITE		0x2
-#define	MEM_F_RW		0x3
-#define	MEM_F_IMMUTABLE		0x4	/* mem_range cannot be unregistered */
-
-void	init_mem(int ncpu);
-int     emulate_mem(struct vcpu *vcpu, uint64_t paddr, struct vie *vie,
-		    struct vm_guest_paging *paging);
+#include <sys/types.h>
+#include <sys/errno.h>
+#include <sys/tree.h>
+#include <machine/vmm.h>
+#include <machine/vmm_instruction_emul.h>
 
-int	read_mem(struct vcpu *vpu, uint64_t gpa, uint64_t *rval, int size);
-int	register_mem(struct mem_range *memp);
-int	register_mem_fallback(struct mem_range *memp);
-int	unregister_mem(struct mem_range *memp);
-int	write_mem(struct vcpu *vcpu, uint64_t gpa, uint64_t wval, int size);
+#include "mem.h"
 
-#endif	/* _MEM_H_ */
+int
+mmio_handle_non_backed_mem(struct vcpu *vcpu __unused, uint64_t paddr __unused,
+    struct mem_range **mr_paramp __unused)
+{
+	return (ESRCH);
+}
diff --git a/usr.sbin/bhyve/riscv/Makefile.inc b/usr.sbin/bhyve/riscv/Makefile.inc
--- a/usr.sbin/bhyve/riscv/Makefile.inc
+++ b/usr.sbin/bhyve/riscv/Makefile.inc
@@ -1,5 +1,6 @@
 SRCS+=			\
-	fdt.c
+	fdt.c		\
+	mem_md.c
 
 .PATH:  ${BHYVE_SYSDIR}/sys/riscv/vmm
 SRCS+=	vmm_instruction_emul.c