xen/arm: ffa: enforce dependency on 4k pages

Adds a BUILD_BUG_ON() to assert the dependency on 4k pages in the FF-A
mediator since the current implementation only works if Xen page size is
the same as the FFA page size.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>

xen/arch/arm/tee/ffa.c

@@ -57,6 +57,16 @@
 #define FFA_MY_VERSION          MAKE_FFA_VERSION(FFA_MY_VERSION_MAJOR, \
                                                  FFA_MY_VERSION_MINOR)
 
+/*
+ * The FF-A specification explicitly works with 4K pages as a measure of
+ * memory size, for example, FFA_RXTX_MAP takes one parameter "RX/TX page
+ * count" which is the number of contiguous 4K pages allocated. Xen may use
+ * a different page size depending on the configuration to avoid confusion
+ * with PAGE_SIZE use a special define when it's a page size as in the FF-A
+ * specification.
+ */
+#define FFA_PAGE_SIZE                   SZ_4K
+
 /*
  * Flags and field values used for the MSG_SEND_DIRECT_REQ/RESP:
  * BIT(31): Framework or partition message
@@ -261,6 +271,17 @@ static bool ffa_probe(void)
     unsigned int major_vers;
     unsigned int minor_vers;
 
+    /*
+     * FF-A often works in units of 4K pages and currently it's assumed
+     * that we can map memory using that granularity. See also the comment
+     * above the FFA_PAGE_SIZE define.
+     *
+     * It is possible to support a PAGE_SIZE larger than 4K in Xen, but
+     * until that is fully handled in this code make sure that we only use
+     * 4K page sizes.
+     */
+    BUILD_BUG_ON(PAGE_SIZE != FFA_PAGE_SIZE);
+
     /*
      * psci_init_smccc() updates this value with what's reported by EL-3
      * or secure world.