diff --git a/exec.c b/exec.c
index 363ec98..997dbb0 100644
--- a/exec.c
+++ b/exec.c
@@ -16,6 +16,34 @@
  * You should have received a copy of the GNU Lesser General Public
  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
+
+/*
+ * Note on memory barriers usage:
+ *
+ * In order for emulated devices "DMA" operations to appear
+ * with a consistent ordering to the guest, we provide some
+ * amount of ordering guarantees:
+ *
+ * cpy_physical_memory_rw() (and all functions using it) is
+ * ordered vs. all previous accesses (it begins with a full
+ * memory barrier)
+ *
+ * This include all the new dma_* accessors.
+ *
+ * The old variants of ld* and st* that have not been convered
+ * to dma_ are not ordered. Users are reponsible for using their
+ * own ordering.
+ *
+ * cpu_physical_memory_map() provides a memory barrier vs. all
+ * previous accesses. There is no implicit barrier on unmap.
+ * If ordering is required between accessed within the map/unmmap
+ * sequence, then it needs to be done explicitely.
+ *
+ * This means that a typical block driver using map/unmap accross
+ * the transfer of a block followed by dma_ writes to signal
+ * completion or interrupt shouldn't require the addition of
+ * explicit barriers.
+ */ 
 #include "config.h"
 #ifdef _WIN32
 #include <windows.h>
@@ -25,6 +53,7 @@
 #endif
 
 #include "qemu-common.h"
+#include "qemu-barrier.h"
 #include "cpu.h"
 #include "tcg.h"
 #include "hw/hw.h"
@@ -3516,6 +3545,10 @@ void cpu_physical_memory_rw(target_phys_addr_t addr, uint8_t *buf,
     target_phys_addr_t page;
     MemoryRegionSection *section;
 
+    /* Provides ordering vs. previous accesses, see comments
+     * at the top of this file */
+    smp_mb();
+
     while (len > 0) {
         page = addr & TARGET_PAGE_MASK;
         l = (page + TARGET_PAGE_SIZE) - addr;
@@ -3713,6 +3746,10 @@ void *cpu_physical_memory_map(target_phys_addr_t addr,
     ram_addr_t rlen;
     void *ret;
 
+    /* Provides ordering vs. previous accesses, see comments
+     * at the top of this file */
+    smp_mb();
+
     while (len > 0) {
         page = addr & TARGET_PAGE_MASK;
         l = (page + TARGET_PAGE_SIZE) - addr;