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(216-180-64-156.dyn.novuscom.net. [216.180.64.156]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2ee0fb10049sm3912861a91.43.2024.11.28.12.15.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Nov 2024 12:15:22 -0800 (PST) From: Pierrick Bouvier To: qemu-devel@nongnu.org Cc: Pierrick Bouvier , "Michael S. Tsirkin" , =?utf-8?q?Philippe_Mathieu-Daud?= =?utf-8?q?=C3=A9?= , Paolo Bonzini , Thomas Huth , =?utf-8?q?Alex_Benn=C3=A9e?= , Mahmoud Mandour , Alexandre Iooss , Markus Armbruster , =?utf-8?q?Marc-Andr=C3=A9_Lureau?= , =?utf-8?q?Daniel_P=2E_Berrang=C3=A9?= , Stefano Garzarella Subject: [PATCH v3 2/3] docs/devel/style: add a section about bitfield, and disallow them for packed structures Date: Thu, 28 Nov 2024 12:15:09 -0800 Message-Id: <20241128201510.869974-3-pierrick.bouvier@linaro.org> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20241128201510.869974-1-pierrick.bouvier@linaro.org> References: <20241128201510.869974-1-pierrick.bouvier@linaro.org> MIME-Version: 1.0 Received-SPF: pass client-ip=2607:f8b0:4864:20::1034; envelope-from=pierrick.bouvier@linaro.org; helo=mail-pj1-x1034.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+patch=linaro.org@nongnu.org Sender: qemu-devel-bounces+patch=linaro.org@nongnu.org Signed-off-by: Pierrick Bouvier Reviewed-by: Richard Henderson Reviewed-by: Philippe Mathieu-Daudé --- docs/devel/style.rst | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/docs/devel/style.rst b/docs/devel/style.rst index 2f68b500798..2d73e6a8f7a 100644 --- a/docs/devel/style.rst +++ b/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 +`_, and on big and +little endian hosts. + +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 +memory usage or usability benefit. + +We encourage the usage of ``include/hw/registerfields.h`` as a safe replacement +for bitfields. + Reserved namespaces in C and POSIX ----------------------------------