docs/devel/style: add a section about bitfield, and disallow them for packed structures

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Tested-by: Stefan Weil <sw@weilnetz.de>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250110203401.178532-3-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250116160306.1709518-25-alex.bennee@linaro.org>

docs/devel/style.rst

@@ -416,6 +416,26 @@ definitions instead of typedefs in headers and function prototypes; this
 avoids problems with duplicated typedefs and reduces the need to include
 headers from other headers.
 
+Bitfields
+---------
+
+C bitfields can be a cause of non-portability issues, especially under windows
+where `MSVC has a different way to lay them out than GCC
+<https://gcc.gnu.org/onlinedocs/gcc/x86-Type-Attributes.html>`_, or where
+endianness matters.
+
+For this reason, we disallow usage of bitfields in packed structures and in any
+structures which are supposed to exactly match a specific layout in guest
+memory. Some existing code may use it, and we carefully ensured the layout was
+the one expected.
+
+We also suggest avoiding bitfields even in structures where the exact
+layout does not matter, unless you can show that they provide a significant
+usability benefit.
+
+We encourage the usage of ``include/hw/registerfields.h`` as a safe replacement
+for bitfields.
+
 Reserved namespaces in C and POSIX
 ----------------------------------