diff --git a/include/exec/memop.h b/include/exec/memop.h
index dfd76a1604..0a610b75d9 100644
--- a/include/exec/memop.h
+++ b/include/exec/memop.h
@@ -12,6 +12,8 @@
 #ifndef MEMOP_H
 #define MEMOP_H
 
+#include "qemu/host-utils.h"
+
 typedef enum MemOp {
     MO_8     = 0,
     MO_16    = 1,
@@ -107,14 +109,20 @@ typedef enum MemOp {
     MO_SSIZE = MO_SIZE | MO_SIGN,
 } MemOp;
 
-/* Size in bytes to MemOp.  */
-static inline unsigned size_memop(unsigned size)
+/* MemOp to size in bytes.  */
+static inline unsigned memop_size(MemOp op)
 {
-    /*
-     * FIXME: No-op to aid conversion of memory_region_dispatch_{read|write}
-     * "unsigned size" operand into a "MemOp op".
-     */
-    return size;
+    return 1 << (op & MO_SIZE);
+}
+
+/* Size in bytes to MemOp.  */
+static inline MemOp size_memop(unsigned size)
+{
+#ifdef CONFIG_DEBUG_TCG
+    /* Power of 2 up to 8.  */
+    assert((size & (size - 1)) == 0 && size >= 1 && size <= 8);
+#endif
+    return ctz32(size);
 }
 
 #endif
diff --git a/include/exec/memory.h b/include/exec/memory.h
index fddc2ff48a..192875b080 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -19,6 +19,7 @@
 #include "exec/cpu-common.h"
 #include "exec/hwaddr.h"
 #include "exec/memattrs.h"
+#include "exec/memop.h"
 #include "exec/ramlist.h"
 #include "qemu/bswap.h"
 #include "qemu/queue.h"
@@ -1749,13 +1750,13 @@ void mtree_info(bool flatview, bool dispatch_tree, bool owner);
  * @mr: #MemoryRegion to access
  * @addr: address within that region
  * @pval: pointer to uint64_t which the data is written to
- * @size: size of the access in bytes
+ * @op: size, sign, and endianness of the memory operation
  * @attrs: memory transaction attributes to use for the access
  */
 MemTxResult memory_region_dispatch_read(MemoryRegion *mr,
                                         hwaddr addr,
                                         uint64_t *pval,
-                                        unsigned size,
+                                        MemOp op,
                                         MemTxAttrs attrs);
 /**
  * memory_region_dispatch_write: perform a write directly to the specified
@@ -1764,13 +1765,13 @@ MemTxResult memory_region_dispatch_read(MemoryRegion *mr,
  * @mr: #MemoryRegion to access
  * @addr: address within that region
  * @data: data to write
- * @size: size of the access in bytes
+ * @op: size, sign, and endianness of the memory operation
  * @attrs: memory transaction attributes to use for the access
  */
 MemTxResult memory_region_dispatch_write(MemoryRegion *mr,
                                          hwaddr addr,
                                          uint64_t data,
-                                         unsigned size,
+                                         MemOp op,
                                          MemTxAttrs attrs);
 
 /**
diff --git a/memory.c b/memory.c
index 7fd93b1d42..3d87908784 100644
--- a/memory.c
+++ b/memory.c
@@ -1446,9 +1446,10 @@ static MemTxResult memory_region_dispatch_read1(MemoryRegion *mr,
 MemTxResult memory_region_dispatch_read(MemoryRegion *mr,
                                         hwaddr addr,
                                         uint64_t *pval,
-                                        unsigned size,
+                                        MemOp op,
                                         MemTxAttrs attrs)
 {
+    unsigned size = memop_size(op);
     MemTxResult r;
 
     if (!memory_region_access_valid(mr, addr, size, false, attrs)) {
@@ -1490,9 +1491,11 @@ static bool memory_region_dispatch_write_eventfds(MemoryRegion *mr,
 MemTxResult memory_region_dispatch_write(MemoryRegion *mr,
                                          hwaddr addr,
                                          uint64_t data,
-                                         unsigned size,
+                                         MemOp op,
                                          MemTxAttrs attrs)
 {
+    unsigned size = memop_size(op);
+
     if (!memory_region_access_valid(mr, addr, size, true, attrs)) {
         unassigned_mem_write(mr, addr, data, size);
         return MEMTX_DECODE_ERROR;
