docs: convert README, CODING_STYLE and HACKING to RST syntax

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

1 files changed, 300 insertions, 0 deletions

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+============
+QEMU Hacking
+============
+
+.. contents:: Table of Contents
+
+Preprocessor
+============
+
+Variadic macros
+---------------
+
+For variadic macros, stick with this C99-like syntax:
+
+.. code-block:: c
+
+    #define DPRINTF(fmt, ...)                                       \
+        do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)
+
+Include directives
+------------------
+
+Order include directives as follows:
+
+.. code-block:: c
+
+    #include "qemu/osdep.h"  /* Always first... */
+    #include <...>           /* then system headers... */
+    #include "..."           /* and finally QEMU headers. */
+
+The "qemu/osdep.h" header contains preprocessor macros that affect the behavior
+of core system headers like <stdint.h>.  It must be the first include so that
+core system headers included by external libraries get the preprocessor macros
+that QEMU depends on.
+
+Do not include "qemu/osdep.h" from header files since the .c file will have
+already included it.
+
+C types
+=======
+
+It should be common sense to use the right type, but we have collected
+a few useful guidelines here.
+
+Scalars
+-------
+
+If you're using "int" or "long", odds are good that there's a better type.
+If a variable is counting something, it should be declared with an
+unsigned type.
+
+If it's host memory-size related, size_t should be a good choice (use
+ssize_t only if required). Guest RAM memory offsets must use ram_addr_t,
+but only for RAM, it may not cover whole guest address space.
+
+If it's file-size related, use off_t.
+If it's file-offset related (i.e., signed), use off_t.
+If it's just counting small numbers use "unsigned int";
+(on all but oddball embedded systems, you can assume that that
+type is at least four bytes wide).
+
+In the event that you require a specific width, use a standard type
+like int32_t, uint32_t, uint64_t, etc.  The specific types are
+mandatory for VMState fields.
+
+Don't use Linux kernel internal types like u32, __u32 or __le32.
+
+Use hwaddr for guest physical addresses except pcibus_t
+for PCI addresses.  In addition, ram_addr_t is a QEMU internal address
+space that maps guest RAM physical addresses into an intermediate
+address space that can map to host virtual address spaces.  Generally
+speaking, the size of guest memory can always fit into ram_addr_t but
+it would not be correct to store an actual guest physical address in a
+ram_addr_t.
+
+For CPU virtual addresses there are several possible types.
+vaddr is the best type to use to hold a CPU virtual address in
+target-independent code. It is guaranteed to be large enough to hold a
+virtual address for any target, and it does not change size from target
+to target. It is always unsigned.
+target_ulong is a type the size of a virtual address on the CPU; this means
+it may be 32 or 64 bits depending on which target is being built. It should
+therefore be used only in target-specific code, and in some
+performance-critical built-per-target core code such as the TLB code.
+There is also a signed version, target_long.
+abi_ulong is for the ``*``-user targets, and represents a type the size of
+'void ``*``' in that target's ABI. (This may not be the same as the size of a
+full CPU virtual address in the case of target ABIs which use 32 bit pointers
+on 64 bit CPUs, like sparc32plus.) Definitions of structures that must match
+the target's ABI must use this type for anything that on the target is defined
+to be an 'unsigned long' or a pointer type.
+There is also a signed version, abi_long.
+
+Of course, take all of the above with a grain of salt.  If you're about
+to use some system interface that requires a type like size_t, pid_t or
+off_t, use matching types for any corresponding variables.
+
+Also, if you try to use e.g., "unsigned int" as a type, and that
+conflicts with the signedness of a related variable, sometimes
+it's best just to use the *wrong* type, if "pulling the thread"
+and fixing all related variables would be too invasive.
+
+Finally, while using descriptive types is important, be careful not to
+go overboard.  If whatever you're doing causes warnings, or requires
+casts, then reconsider or ask for help.
+
+Pointers
+--------
+
+Ensure that all of your pointers are "const-correct".
+Unless a pointer is used to modify the pointed-to storage,
+give it the "const" attribute.  That way, the reader knows
+up-front that this is a read-only pointer.  Perhaps more
+importantly, if we're diligent about this, when you see a non-const
+pointer, you're guaranteed that it is used to modify the storage
+it points to, or it is aliased to another pointer that is.
+
+Typedefs
+--------
+
+Typedefs are used to eliminate the redundant 'struct' keyword, since type
+names have a different style than other identifiers ("CamelCase" versus
+"snake_case").  Each named struct type should have a CamelCase name and a
+corresponding typedef.
+
+Since certain C compilers choke on duplicated typedefs, you should avoid
+them and declare a typedef only in one header file.  For common types,
+you can use "include/qemu/typedefs.h" for example.  However, as a matter
+of convenience it is also perfectly fine to use forward struct
+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.
+
+Reserved namespaces in C and POSIX
+----------------------------------
+
+Underscore capital, double underscore, and underscore 't' suffixes should be
+avoided.
+
+Low level memory management
+===========================
+
+Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign
+APIs is not allowed in the QEMU codebase. Instead of these routines,
+use the GLib memory allocation routines g_malloc/g_malloc0/g_new/
+g_new0/g_realloc/g_free or QEMU's qemu_memalign/qemu_blockalign/qemu_vfree
+APIs.
+
+Please note that g_malloc will exit on allocation failure, so there
+is no need to test for failure (as you would have to with malloc).
+Calling g_malloc with a zero size is valid and will return NULL.
+
+Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the following
+reasons:
+
+* It catches multiplication overflowing size_t;
+* It returns T ``*`` instead of void ``*``, letting compiler catch more type errors.
+
+Declarations like
+
+.. code-block:: c
+
+    T *v = g_malloc(sizeof(*v))
+
+are acceptable, though.
+
+Memory allocated by qemu_memalign or qemu_blockalign must be freed with
+qemu_vfree, since breaking this will cause problems on Win32.
+
+String manipulation
+===================
+
+Do not use the strncpy function.  As mentioned in the man page, it does *not*
+guarantee a NULL-terminated buffer, which makes it extremely dangerous to use.
+It also zeros trailing destination bytes out to the specified length.  Instead,
+use this similar function when possible, but note its different signature:
+
+.. code-block:: c
+
+    void pstrcpy(char *dest, int dest_buf_size, const char *src)
+
+Don't use strcat because it can't check for buffer overflows, but:
+
+.. code-block:: c
+
+    char *pstrcat(char *buf, int buf_size, const char *s)
+
+The same limitation exists with sprintf and vsprintf, so use snprintf and
+vsnprintf.
+
+QEMU provides other useful string functions:
+
+.. code-block:: c
+
+    int strstart(const char *str, const char *val, const char **ptr)
+    int stristart(const char *str, const char *val, const char **ptr)
+    int qemu_strnlen(const char *s, int max_len)
+
+There are also replacement character processing macros for isxyz and toxyz,
+so instead of e.g. isalnum you should use qemu_isalnum.
+
+Because of the memory management rules, you must use g_strdup/g_strndup
+instead of plain strdup/strndup.
+
+Printf-style functions
+======================
+
+Whenever you add a new printf-style function, i.e., one with a format
+string argument and following "..." in its prototype, be sure to use
+gcc's printf attribute directive in the prototype.
+
+This makes it so gcc's -Wformat and -Wformat-security options can do
+their jobs and cross-check format strings with the number and types
+of arguments.
+
+C standard, implementation defined and undefined behaviors
+==========================================================
+
+C code in QEMU should be written to the C99 language specification. A copy
+of the final version of the C99 standard with corrigenda TC1, TC2, and TC3
+included, formatted as a draft, can be downloaded from:
+
+    `<http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf>`_
+
+The C language specification defines regions of undefined behavior and
+implementation defined behavior (to give compiler authors enough leeway to
+produce better code).  In general, code in QEMU should follow the language
+specification and avoid both undefined and implementation defined
+constructs. ("It works fine on the gcc I tested it with" is not a valid
+argument...) However there are a few areas where we allow ourselves to
+assume certain behaviors because in practice all the platforms we care about
+behave in the same way and writing strictly conformant code would be
+painful. These are:
+
+* you may assume that integers are 2s complement representation
+* you may assume that right shift of a signed integer duplicates
+  the sign bit (ie it is an arithmetic shift, not a logical shift)
+
+In addition, QEMU assumes that the compiler does not use the latitude
+given in C99 and C11 to treat aspects of signed '<<' as undefined, as
+documented in the GNU Compiler Collection manual starting at version 4.0.
+
+Error handling and reporting
+============================
+
+Reporting errors to the human user
+----------------------------------
+
+Do not use printf(), fprintf() or monitor_printf().  Instead, use
+error_report() or error_vreport() from error-report.h.  This ensures the
+error is reported in the right place (current monitor or stderr), and in
+a uniform format.
+
+Use error_printf() & friends to print additional information.
+
+error_report() prints the current location.  In certain common cases
+like command line parsing, the current location is tracked
+automatically.  To manipulate it manually, use the loc_``*``() from
+error-report.h.
+
+Propagating errors
+------------------
+
+An error can't always be reported to the user right where it's detected,
+but often needs to be propagated up the call chain to a place that can
+handle it.  This can be done in various ways.
+
+The most flexible one is Error objects.  See error.h for usage
+information.
+
+Use the simplest suitable method to communicate success / failure to
+callers.  Stick to common methods: non-negative on success / -1 on
+error, non-negative / -errno, non-null / null, or Error objects.
+
+Example: when a function returns a non-null pointer on success, and it
+can fail only in one way (as far as the caller is concerned), returning
+null on failure is just fine, and certainly simpler and a lot easier on
+the eyes than propagating an Error object through an Error ``*````*`` parameter.
+
+Example: when a function's callers need to report details on failure
+only the function really knows, use Error ``*````*``, and set suitable errors.
+
+Do not report an error to the user when you're also returning an error
+for somebody else to handle.  Leave the reporting to the place that
+consumes the error returned.
+
+Handling errors
+---------------
+
+Calling exit() is fine when handling configuration errors during
+startup.  It's problematic during normal operation.  In particular,
+monitor commands should never exit().
+
+Do not call exit() or abort() to handle an error that can be triggered
+by the guest (e.g., some unimplemented corner case in guest code
+translation or device emulation).  Guests should not be able to
+terminate QEMU.
+
+Note that &error_fatal is just another way to exit(1), and &error_abort
+is just another way to abort().