diff --git a/tools/libxc/include/xc_dom.h b/tools/libxc/include/xc_dom.h
index cac4698..e5ab56c 100644
--- a/tools/libxc/include/xc_dom.h
+++ b/tools/libxc/include/xc_dom.h
@@ -216,6 +216,34 @@ struct xc_dom_image {
     struct xc_hvm_firmware_module smbios_module;
 };
 
+#if defined(__i386__) || defined(__x86_64__)
+/* C representation of the x86/HVM start info layout.
+ *
+ * The canonical definition of this layout resides in public/xen.h, this
+ * is just a way to represent the layout described there using C types.
+ *
+ * NB: the packed attribute is not really needed, but it helps us enforce
+ * the fact this this is just a representation, and it might indeed
+ * be required in the future if there are alignment changes.
+ */
+struct hvm_start_info {
+    uint32_t magic;             /* Contains the magic value 0x336ec578       */
+                                /* ("xEn3" with the 0x80 bit of the "E" set).*/
+    uint32_t flags;             /* SIF_xxx flags.                            */
+    uint32_t cmdline_paddr;     /* Physical address of the command line.     */
+    uint32_t nr_modules;        /* Number of modules passed to the kernel.   */
+    uint32_t modlist_paddr;     /* Physical address of an array of           */
+                                /* hvm_modlist_entry.                        */
+} __attribute__((packed));
+
+struct hvm_modlist_entry {
+    uint32_t paddr;             /* Physical address of the module.           */
+    uint32_t size;              /* Size of the module in bytes.              */
+    uint32_t cmdline_paddr;     /* Physical address of the command line.     */
+    uint32_t reserved;
+} __attribute__((packed));
+#endif /* x86 */
+
 /* --- pluggable kernel loader ------------------------------------- */
 
 struct xc_dom_loader {
diff --git a/xen/include/public/xen.h b/xen/include/public/xen.h
index 7b629b1..e1350d0 100644
--- a/xen/include/public/xen.h
+++ b/xen/include/public/xen.h
@@ -790,22 +790,36 @@ typedef struct start_info start_info_t;
  * NOTE: nothing will be loaded at physical address 0, so
  * a 0 value in any of the address fields should be treated
  * as not present.
+ *
+ *  0 +----------------+
+ *    | magic          | Contains the magic value HVM_START_MAGIC_VALUE
+ *    |                | ("xEn3" with the 0x80 bit of the "E" set).
+ *  4 +----------------+
+ *    | flags          | SIF_xxx flags.
+ *  8 +----------------+
+ *    | cmdline_paddr  | Physical address of the command line,
+ *    |                | a zero-terminated ASCII string.
+ * 12 +----------------+
+ *    | nr_modules     | Number of modules passed to the kernel.
+ * 16 +----------------+
+ *    | modlist_paddr  | Physical address of an array of modules
+ *    |                | (layout of the structure below).
+ * 20 +----------------+
+ *
+ * The layout of each entry in the module structure is the following:
+ *
+ *  0 +----------------+
+ *    | paddr          | Physical address of the module.
+ *  4 +----------------+
+ *    | size           | Size of the module in bytes.
+ *  8 +----------------+
+ *    | cmdline_paddr  | Physical address of the command line,
+ *    |                | a zero-terminated ASCII string.
+ * 12 +----------------+
+ *    | reserved       |
+ * 16 +----------------+
  */
-struct hvm_start_info {
 #define HVM_START_MAGIC_VALUE 0x336ec578
-    uint32_t magic;             /* Contains the magic value 0x336ec578       */
-                                /* ("xEn3" with the 0x80 bit of the "E" set).*/
-    uint32_t flags;             /* SIF_xxx flags.                            */
-    uint32_t cmdline_paddr;     /* Physical address of the command line.     */
-    uint32_t nr_modules;        /* Number of modules passed to the kernel.   */
-    uint32_t modlist_paddr;     /* Physical address of an array of           */
-                                /* hvm_modlist_entry.                        */
-};
-
-struct hvm_modlist_entry {
-    uint32_t paddr;             /* Physical address of the module.           */
-    uint32_t size;              /* Size of the module in bytes.              */
-};
 
 /* New console union for dom0 introduced in 0x00030203. */
 #if __XEN_INTERFACE_VERSION__ < 0x00030203
