[multi-arch-dev] doc: Add initial multi-arch-coding-guideline

Tracked-On: #8782
Signed-off-by: Yifan Liu <yifan1.liu@intel.com>

Author: yfliuuu

multi-arch-coding-guideline.rst

@@ -0,0 +1,334 @@
+Multi-architecture development coding guideline
+###############################################
+
+This document is intended for guiding initial development of
+ACRN multi-architecture.
+
+This document is NOT a coding style guideline (where one normally expects
+that the document explains things such as indentation, naming convention,
+typedef rules, etc.)
+
+During early development of multi-architecture project, the APIs
+and data structures should be organized in a consistent approach for
+better readability and maintainability. There are multiple ways to organize
+these factors and this documentation lays out a guideline to assist future
+development.
+
+General Assumptions and Practices
+*********************************
+
+Due to FuSa constraints, we have several compiler features or coding styles that
+are NOT encouraged in ``hypervisor/`` code:
+
+* Weak link (``__weak``): There are several MISRA rules that discourage uses of weak
+  link (Rule 1.2, 5.3, 5.8, 5.9). See `coding-guidelines`_ for more information.
+
+* Function-like macro: Use inline function whenever possible. Function-like macros
+  should be avoided unless absolutely necessary. This also implies that section-based
+  variable placement is also discouraged (utilizes linker to group symbols in certain
+  binary sections. See ``devicemodel/include/types.h``, macro ``SET_*``).
+
+Practices and general rule of thumb:
+
+* Enable compiler ``-O2`` as default.
+
+* APIs that are public (globally visible) but implemented in arch domains should start
+  their name with ``arch_``. If this API is optional, a default/empty ``arch_`` API
+  can be implemented in common C/header file.
+
+* Architecture headers should be exposure-controlled: If a structure or an API is used/shared
+  only within the module, then do not expose it to public headers, put it to private headers.
+  There is also a reverse statement: all arch symbols exposed to common scope should be public.
+  This is nearly impossible in practice. So we follow only the former part, and leave the latter
+  as future optimization.
+
+.. _coding-guidelines: https://projectacrn.github.io/latest/developer-guides/coding_guidelines.html
+
+Data Structures
+***************
+
+In multi-architecture project we might have a common data structure that needs
+to carry some architecture specific information (for example, a vCPU object).
+
+For this type of data structures we prefer the following style:
+
+In common header:
+
+.. code-block:: c
+
+   /* File: common.h */
+   #include <arch-public.h>
+
+   struct foo {
+       <common member1>;
+       <common member2>;
+       <common member3>;
+       struct arch_foo arch;
+   };
+
+   struct bar {
+       <common member1>;
+       <common member2>;
+       <common member3>;
+   }
+
+And in architecture specific header:
+
+.. code-block:: c
+
+   /* File: arch-public.h */
+
+   /* do NOT include common.h here */
+
+   struct arch_foo {
+       <arch member1>;
+       <arch member2>;
+       <arch member3>;
+   };
+
+   /* Forward declaration when the reference of struct is opaque */
+   struct foo;
+   struct bar;
+
+   struct baz {
+       struct foo *f;
+       struct bar *b;
+   }
+
+   /* Forward declaration is also needed for function argument pointer */
+   void some_function(struct foo *);
+
+The architecture public header should NOT in turn include common header
+that already included the public header.
+
+Consider an inclusion graph with nodes being headers, and edges being
+inclusions. If A includes B, there's an edge from A to B. Ideally, the
+header inclusion graph should be **acyclic**.
+
+If a downstream header needs something from upstream headers, (upstream and
+downstream refers to positions relative to inclusion chain) a forward declaration
+SHOULD be sufficient. If not, then there are problems with your organization of headers.
+
+However in practice, people will not spend time following inclusion chain to
+check if it is acyclic, and this issue is often noticable when there's a circular
+dependency compilation error. In most projects (especially large ones), as long
+as it compiles, it is OK.
+
+So in our project, the **acyclic inclusion** is not an enforcement, but a
+practice that is strongly advised.
+
+Interfaces
+**********
+
+Here we consider only common-arch interfaces. Interfaces within each architecture
+are out of scope of this section.
+
+There are several form of interfaces used in different use cases.
+
+Function prototypes
+===================
+
+This is the most common form of interface where a function is referenced in common scope
+and implemented in arch-specific scope.
+
+We prefer the following style:
+
+In common header:
+
+.. code-block:: c
+
+   /* File: common.h */
+
+   #include <arch-public.h>
+
+   /*
+    * Each architecture MUST implement this API in arch C file.
+    * The implementation SHOULD NOT be marked as static inline
+    * even if the implementation is small.
+    */
+   void arch_api_mandatory(void);
+
+   /*
+    * Short helpers that each architecture MAY provide.
+    * Due to FuSa constraints described above,
+    * Use a static inline function instead of a macro
+    * to select arch implementation.
+    */
+   static inline void quick_helper(void) { arch_helper(); }
+
+   /*
+    * Architecture MAY implement this API.
+    * If it does not, an empty stub is provided.
+    * Define ARCH_HAS_SOME_CAPABILITY in arch headers to
+    * indicate implementation.
+    */
+   #ifdef ARCH_HAS_SOME_CAPABILITY
+   int arch_api_optional(void);
+   #else
+   static inline int arch_api_optional(void) { return 0; }
+   #endif
+
+   /*
+    * Architecture MAY implement this API.
+    * If it does not, a common default is provided in common C file.
+    * Due to FuSa constraint described above, __weak is discouraged.
+    * Define ARCH_HAS_OPTIONAL_WITH_DEFAULT in arch header to
+    * indicate implementation instead.
+    */
+   void arch_api_optional_with_default(void);
+
+   /*
+    * Configuration-based APIs
+    * The CONFIG macro is provided by header generated by build-system.
+    */
+   #ifdef CONFIG_SWITCH_1
+   void arch_api_conditional(void);
+   #endif
+
+In common C file:
+
+.. code-block:: c
+
+   /* File: common.c */
+
+   #include <common.h>
+
+   #ifndef ARCH_HAS_OPTIONAL_WITH_DEFAULT
+   void arch_api_optional_with_default(void) {
+       /* default implementation */
+   }
+   #endif
+
+   void common_logic(void) {
+       arch_api_mandatory();
+
+       arch_api_optional();
+
+       arch_api_optional_with_default();
+
+   #ifdef CONFIG_SWITCH_1
+       arch_api_conditional();
+   #endif
+   }
+
+In architecture-specific public header file:
+
+.. code-block:: c
+
+   /* File: arch-public.h */
+
+   /* Indicates implementation to optional API */
+   #define ARCH_HAS_SOME_CAPABILITY
+
+   /* Indicates implementation to optional_with_default API */
+   #define ARCH_HAS_OPTIONAL_WITH_DEFAULT
+
+   static inline void arch_helper(void) {
+       /* short implementation */
+   }
+
+In architecture-specific C file:
+
+.. code-block:: c
+
+   /* File: arch.c */
+   #include <common.h>
+   #include <arch-priate.h>
+
+   void arch_api_mandatory(void) {
+       /* implementation */
+   }
+
+   /* ARCH_HAS_SOME_CAPABILITY is defined in arch-public header */
+   void arch_api_optional(void) {
+       /* implementation */
+   }
+
+   /* ARCH_HAS_OPTIONAL_WITH_DEFAULT is defined in arch-public header */
+   void arch_api_optional_with_default(void) {
+       /* implementation */
+   }
+
+   #ifdef CONFIG_SWITCH_1
+   void arch_api_conditional(void) {
+       /* implementation */
+   }
+   #endif
+
+Register Callbacks
+==================
+
+Callbacks are used when we do NOT know which implementation to be used
+until runtime. To determine implementation we need to do either detection,
+or accepting user input.
+
+With this approach, all implementations will be compiled-in, and each implementation
+exposes a callback data structure typically called ``*_ops`` to common scope.
+
+In common header file:
+
+.. code-block:: c
+
+   /* File: common.h */
+   struct foo_ops {
+       void (*op1)(void);
+       int (*op2)(int);
+       int (*op3)(struct bar *);
+   };
+
+   extern struct foo_ops a_ops;
+   extern struct foo_ops b_ops;
+
+
+In common C file:
+
+.. code-block:: c
+
+   /* File: common.c */
+
+   #include <common.h>
+
+   void do_detection() {
+       /* detects environment */
+       if (condition_a) {
+           register_callback(a_ops);
+       } else {
+           register_callback(b_ops);
+       }
+   }
+
+   void actual_call() {
+       op->op1();
+       op->op2(1);
+   }
+
+In arch header file:
+
+.. code-block:: c
+
+   /* File: arch-public.h */
+
+   /* Do nothing */
+
+.. code-block:: c
+
+   /* File: arch.c */
+   #include <common.h>
+
+   static void a_op1(void) {
+       /* implementation */
+   }
+
+   static int a_op2(void) {
+       /* implementation */
+   }
+
+   static int a_op2(struct bar *b) {
+       /* implementation */
+   }
+
+   const struct foo_ops a_ops = {
+       .op1 = a_op1,
+       .op2 = a_op2,
+       .op3 = a_op3,
+   }