diff --git a/sys/conf/files b/sys/conf/files
--- a/sys/conf/files
+++ b/sys/conf/files
@@ -3437,6 +3437,7 @@
 dev/virtio/pci/virtio_pci_modern.c	optional	virtio_pci
 dev/virtio/mmio/virtio_mmio.c		optional	virtio_mmio
 dev/virtio/mmio/virtio_mmio_acpi.c	optional	virtio_mmio acpi
+dev/virtio/mmio/virtio_mmio_cmdline.c	optional	virtio_mmio
 dev/virtio/mmio/virtio_mmio_fdt.c	optional	virtio_mmio fdt
 dev/virtio/mmio/virtio_mmio_if.m	optional	virtio_mmio
 dev/virtio/network/if_vtnet.c		optional	vtnet
diff --git a/sys/dev/virtio/mmio/virtio_mmio_cmdline.c b/sys/dev/virtio/mmio/virtio_mmio_cmdline.c
new file mode 100644
--- /dev/null
+++ b/sys/dev/virtio/mmio/virtio_mmio_cmdline.c
@@ -0,0 +1,150 @@
+/*-
+ * Copyright (c) 2022 Colin Percival
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    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 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 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)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/param.h>
+#include <sys/systm.h>
+#include <sys/bus.h>
+#include <sys/kernel.h>
+#include <sys/limits.h>
+#include <sys/malloc.h>
+#include <sys/module.h>
+#include <sys/rman.h>
+
+#include <machine/bus.h>
+#include <machine/resource.h>
+
+#include <dev/virtio/mmio/virtio_mmio.h>
+
+/* Parse <size>@<baseaddr>:<irq>[:<id>] and add a child. */
+static void
+parsearg(driver_t *driver, device_t parent, char * arg)
+{
+	device_t child;
+	char * p;
+	unsigned long sz;
+	unsigned long baseaddr;
+	unsigned long irq;
+	unsigned long id;
+
+	/* <size> */
+	sz = strtoul(arg, &p, 0);
+	if ((sz == 0) || (sz == ULONG_MAX))
+		goto bad;
+	switch (*p) {
+	case 'E': case 'e':
+		sz <<= 10;
+		/* FALLTHROUGH */
+	case 'P': case 'p':
+		sz <<= 10;
+		/* FALLTHROUGH */
+	case 'T': case 't':
+		sz <<= 10;
+		/* FALLTHROUGH */
+	case 'G': case 'g':
+		sz <<= 10;
+		/* FALLTHROUGH */
+	case 'M': case 'm':
+		sz <<= 10;
+		/* FALLTHROUGH */
+	case 'K': case 'k':
+		sz <<= 10;
+		p++;
+		break;
+	}
+
+	/* @<baseaddr> */
+	if (*p++ != '@')
+		goto bad;
+	baseaddr = strtoul(p, &p, 0);
+	if ((baseaddr == 0) || (baseaddr == ULONG_MAX))
+		goto bad;
+
+	/* :<irq> */
+	if (*p++ != ':')
+		goto bad;
+	irq = strtoul(p, &p, 0);
+	if ((irq == 0) || (irq == ULONG_MAX))
+		goto bad;
+
+	/* Optionally, :<id> */
+	if (*p) {
+		if (*p++ != ':')
+			goto bad;
+		id = strtoul(p, &p, 0);
+		if ((id == 0) || (id == ULONG_MAX))
+			goto bad;
+	} else {
+		id = 0;
+	}
+
+	/* Should have reached the end of the string. */
+	if (*p)
+		goto bad;
+
+	/* Create the child and assign its resources. */
+	child = BUS_ADD_CHILD(parent, 0, driver->name, id ? id : -1);
+	bus_set_resource(child, SYS_RES_MEMORY, 0, baseaddr, sz);
+	bus_set_resource(child, SYS_RES_IRQ, 0, irq, 1);
+	device_set_driver(child, driver);
+
+	return;
+
+bad:
+	printf("Error parsing virtio_mmio parameter: %s\n", arg);
+}
+
+static void
+vtmmio_cmdline_identify(driver_t *driver, device_t parent)
+{
+	size_t n;
+	char name[] = "virtio_mmio.device_XXXX";
+	char * val;
+
+	/* First variable just has its own name. */
+	if ((val = kern_getenv("virtio_mmio.device")) == NULL)
+		return;
+	parsearg(driver, parent, val);
+	freeenv(val);
+
+	/* The rest have _%zu suffixes. */
+	for (n = 1; n <= 9999; n++) {
+		sprintf(name, "virtio_mmio.device_%zu", n);
+		if ((val = kern_getenv(name)) == NULL)
+			return;
+		parsearg(driver, parent, val);
+		freeenv(val);
+	}
+}
+
+static device_method_t vtmmio_cmdline_methods[] = {
+        /* Device interface. */
+	DEVMETHOD(device_identify,	vtmmio_cmdline_identify),
+	DEVMETHOD(device_probe,		vtmmio_probe),
+
+        DEVMETHOD_END
+};
+DEFINE_CLASS_1(virtio_mmio, vtmmio_cmdline_driver, vtmmio_cmdline_methods,
+    sizeof(struct vtmmio_softc), vtmmio_driver);
+DRIVER_MODULE(vtmmio_cmdline, nexus, vtmmio_cmdline_driver, 0, 0);