diff --git a/sys/dev/virtio/mmio/virtio_mmio.c b/sys/dev/virtio/mmio/virtio_mmio.c
--- a/sys/dev/virtio/mmio/virtio_mmio.c
+++ b/sys/dev/virtio/mmio/virtio_mmio.c
@@ -88,7 +88,7 @@
 static uint8_t	vtmmio_get_status(device_t);
 static void	vtmmio_set_status(device_t, uint8_t);
 static void	vtmmio_read_dev_config(device_t, bus_size_t, void *, int);
-static void	vtmmio_write_dev_config(device_t, bus_size_t, void *, int);
+static void	vtmmio_write_dev_config(device_t, bus_size_t, const void *, int);
 static void	vtmmio_describe_features(struct vtmmio_softc *, const char *,
 		    uint64_t);
 static void	vtmmio_probe_and_attach_child(struct vtmmio_softc *);
@@ -737,7 +737,7 @@
 
 static void
 vtmmio_write_dev_config(device_t dev, bus_size_t offset,
-    void *src, int length)
+    const void *src, int length)
 {
 	struct vtmmio_softc *sc;
 	bus_size_t off;
@@ -755,21 +755,21 @@
 	if (sc->vtmmio_version > 1) {
 		switch (length) {
 		case 1:
-			vtmmio_write_config_1(sc, off, *(uint8_t *)src);
+			vtmmio_write_config_1(sc, off, *(const uint8_t *)src);
 			break;
 		case 2:
 			vtmmio_write_config_2(sc, off,
-			    htole16(*(uint16_t *)src));
+			    htole16(*(const uint16_t *)src));
 			break;
 		case 4:
 			vtmmio_write_config_4(sc, off,
-			    htole32(*(uint32_t *)src));
+			    htole32(*(const uint32_t *)src));
 			break;
 		case 8:
 			vtmmio_write_config_4(sc, off,
-			    htole32(*(uint64_t *)src));
+			    htole32(*(const uint64_t *)src));
 			vtmmio_write_config_4(sc, off + 4,
-			    htole32((*(uint64_t *)src) >> 32));
+			    htole32((*(const uint64_t *)src) >> 32));
 			break;
 		default:
 			panic("%s: invalid length %d\n", __func__, length);
diff --git a/sys/dev/virtio/pci/virtio_pci_legacy.c b/sys/dev/virtio/pci/virtio_pci_legacy.c
--- a/sys/dev/virtio/pci/virtio_pci_legacy.c
+++ b/sys/dev/virtio/pci/virtio_pci_legacy.c
@@ -97,7 +97,7 @@
 static void	vtpci_legacy_reinit_complete(device_t);
 static void	vtpci_legacy_notify_vq(device_t, uint16_t, bus_size_t);
 static void	vtpci_legacy_read_dev_config(device_t, bus_size_t, void *, int);
-static void	vtpci_legacy_write_dev_config(device_t, bus_size_t, void *, int);
+static void	vtpci_legacy_write_dev_config(device_t, bus_size_t, const void *, int);
 
 static bool	vtpci_legacy_setup_msix(struct vtpci_legacy_softc *sc);
 static void	vtpci_legacy_teardown_msix(struct vtpci_legacy_softc *sc);
@@ -519,11 +519,11 @@
 
 static void
 vtpci_legacy_write_dev_config(device_t dev, bus_size_t offset,
-    void *src, int length)
+    const void *src, int length)
 {
 	struct vtpci_legacy_softc *sc;
 	bus_size_t off;
-	uint8_t *s;
+	const uint8_t *s;
 	int i;
 
 	sc = device_get_softc(dev);
diff --git a/sys/dev/virtio/pci/virtio_pci_modern.c b/sys/dev/virtio/pci/virtio_pci_modern.c
--- a/sys/dev/virtio/pci/virtio_pci_modern.c
+++ b/sys/dev/virtio/pci/virtio_pci_modern.c
@@ -123,7 +123,7 @@
 static void	vtpci_modern_notify_vq(device_t, uint16_t, bus_size_t);
 static int	vtpci_modern_config_generation(device_t);
 static void	vtpci_modern_read_dev_config(device_t, bus_size_t, void *, int);
-static void	vtpci_modern_write_dev_config(device_t, bus_size_t, void *, int);
+static void	vtpci_modern_write_dev_config(device_t, bus_size_t, const void *, int);
 
 static int	vtpci_modern_probe_configs(device_t);
 static int	vtpci_modern_find_cap(device_t, uint8_t, int *);
@@ -686,7 +686,7 @@
 }
 
 static void
-vtpci_modern_write_dev_config(device_t dev, bus_size_t offset, void *src,
+vtpci_modern_write_dev_config(device_t dev, bus_size_t offset, const void *src,
     int length)
 {
 	struct vtpci_modern_softc *sc;
@@ -700,20 +700,20 @@
 
 	switch (length) {
 	case 1:
-		vtpci_modern_write_device_1(sc, offset, *(uint8_t *) src);
+		vtpci_modern_write_device_1(sc, offset, *(const uint8_t *) src);
 		break;
 	case 2: {
-		uint16_t val = virtio_gtoh16(true, *(uint16_t *) src);
+		uint16_t val = virtio_gtoh16(true, *(const uint16_t *) src);
 		vtpci_modern_write_device_2(sc, offset, val);
 		break;
 	}
 	case 4: {
-		uint32_t val = virtio_gtoh32(true, *(uint32_t *) src);
+		uint32_t val = virtio_gtoh32(true, *(const uint32_t *) src);
 		vtpci_modern_write_device_4(sc, offset, val);
 		break;
 	}
 	case 8: {
-		uint64_t val = virtio_gtoh64(true, *(uint64_t *) src);
+		uint64_t val = virtio_gtoh64(true, *(const uint64_t *) src);
 		vtpci_modern_write_device_8(sc, offset, val);
 		break;
 	}
diff --git a/sys/dev/virtio/virtio.h b/sys/dev/virtio/virtio.h
--- a/sys/dev/virtio/virtio.h
+++ b/sys/dev/virtio/virtio.h
@@ -123,7 +123,7 @@
 void	 virtio_read_device_config(device_t dev, bus_size_t offset,
 	     void *dst, int length);
 void	 virtio_write_device_config(device_t dev, bus_size_t offset,
-	     void *src, int length);
+	     const void *src, int length);
 
 /* Inlined device specific read/write functions for common lengths. */
 #define VIRTIO_RDWR_DEVICE_CONFIG(size, type)				\
diff --git a/sys/dev/virtio/virtio.c b/sys/dev/virtio/virtio.c
--- a/sys/dev/virtio/virtio.c
+++ b/sys/dev/virtio/virtio.c
@@ -321,7 +321,7 @@
 }
 
 void
-virtio_write_device_config(device_t dev, bus_size_t offset, void *dst, int len)
+virtio_write_device_config(device_t dev, bus_size_t offset, const void *dst, int len)
 {
 
 	VIRTIO_BUS_WRITE_DEVICE_CONFIG(device_get_parent(dev),
diff --git a/sys/dev/virtio/virtio_bus_if.m b/sys/dev/virtio/virtio_bus_if.m
--- a/sys/dev/virtio/virtio_bus_if.m
+++ b/sys/dev/virtio/virtio_bus_if.m
@@ -107,7 +107,7 @@
 METHOD void write_device_config {
 	device_t	dev;
 	bus_size_t	offset;
-	void		*src;
+	const void	*src;
 	int		len;
 };