Profiler: Highlight profiling platforms

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: I3211a8d4e744b2fe94f492f03919b16792ef2900
diff --git a/docs/profiling.rst b/docs/profiling.rst
index e913b75..18fdf7d 100644
--- a/docs/profiling.rst
+++ b/docs/profiling.rst
@@ -46,6 +46,12 @@
         -DTFM_TOOLCHAIN_FILE=build/spe/api_ns/cmake/toolchain_ns_GNUARM.cmake
   cmake --build build/nspe -- -j
 
+.. Note::
+
+    TF-M profiling implementation relies on the physical CPU cycles provided by hardware
+    timer (refer to `Implement the HAL`_). It may not be supported on virtual platforms
+    or emulators.
+
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 Profiler Integration Reference
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