Index: sys/dev/nvme/nvme_private.h
===================================================================
--- sys/dev/nvme/nvme_private.h	(revision 338687)
+++ sys/dev/nvme/nvme_private.h	(working copy)
@@ -331,13 +333,17 @@
 #define nvme_mmio_offsetof(reg)						       \
 	offsetof(struct nvme_registers, reg)
 
-#define nvme_mmio_read_4(sc, reg)					       \
+#define nvme_mmio_read_4(sc, reg) ({					       \
+	bus_space_barrier((sc)->bus_tag, (sc)->bus_handle,		       \
+	    nvme_mmio_offsetof(reg), 4, BUS_SPACE_BARRIER_READ);	       \
 	bus_space_read_4((sc)->bus_tag, (sc)->bus_handle,		       \
-	    nvme_mmio_offsetof(reg))
+	    nvme_mmio_offsetof(reg)); })
 
 #define nvme_mmio_write_4(sc, reg, val)					       \
 	bus_space_write_4((sc)->bus_tag, (sc)->bus_handle,		       \
-	    nvme_mmio_offsetof(reg), val)
+	    nvme_mmio_offsetof(reg), val);				       \
+	bus_space_barrier((sc)->bus_tag, (sc)->bus_handle,		       \
+	    nvme_mmio_offsetof(reg), 4, BUS_SPACE_BARRIER_WRITE)
 
 #define nvme_mmio_write_8(sc, reg, val)					       \
 	do {								       \
@@ -346,6 +352,8 @@
 		bus_space_write_4((sc)->bus_tag, (sc)->bus_handle,	       \
 		    nvme_mmio_offsetof(reg)+4,				       \
 		    (val & 0xFFFFFFFF00000000ULL) >> 32);		       \
+		bus_space_barrier((sc)->bus_tag, (sc)->bus_handle,	       \
+		    nvme_mmio_offsetof(reg), 8, BUS_SPACE_BARRIER_WRITE);      \
 	} while (0);
 
 #if __FreeBSD_version < 800054