diff --git a/docs/porting-guide.md b/docs/porting-guide.md
index 56ddbb1c1521fefa19fe03e24ff5ced37b040558..d19811d19f00f56aafd6064427ee6c9ffb2a49f8 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -642,7 +642,8 @@ CPU-specific linear index into blocks of memory (for example while allocating
 per-CPU stacks). This function will be invoked very early in the
 initialization sequence which mandates that this function should be
 implemented in assembly and should not rely on the avalability of a C
-runtime environment.
+runtime environment. This function can clobber x0 - x8 and must preserve
+x9 - x29.
 
 This function plays a crucial role in the power domain topology framework in
 PSCI and details of this can be found in [Power Domain Topology Design].