LuaJIT 2.0 is a just-in-time compiler for the Lua scripting language; it is the preferred way to embed Lua in a plugin. This technote describes the integration issues between X-Plane 10.20 64-bit and LuaJIT. These problems are specific to 64-bit builds of X-Plane; 32-bit plugins and Windows/Linux 64-bit plugins are not affected.

LuaJIT 2.0 and Address Space

LuaJIT 2.0 requires that all allocations used by JIT code be in the bottom 2 GB of address space. (This requirement comes from the use of signed 32-bit relative addressing for constants in the generated code.) To meet this requirement, LuaJIT has two modifications from a ‘standard’ Lua run-time:

  • The built-in Lua allocator attempts to grab memory from the lower 2 GB of address space using various OS-specific techniques. The allocator is a copy of dl-malloc sitting on top of this custom bottom-2-gb scheme.
  • lua_newstate is inoperative in the 64-bit version of LuaJIT 2.0. This stops client code from providing a custom allocator that ignores the < 2 GB rule (which would cause JIT-code crashes).

LuaJIT 2.0 and X-Plane

The problem: during normal operation, X-Plane’s normal memory use may consume some or all of the free memory in the bottom 2 GB, potentially exhausting it under high scenery engine load. Plugins utilizing Lua that are loaded late (e.g. in an airplane load) or need more memory mid-run may fail allocation since this critical “bottom 2 GB” of address space is taken.

X-Plane 64-bit sometimes solves this problem by pre-grabbing the entire LuaJIT address space for itself on startup and then providing this address space to Lua-based plugins on demand. A special set of messages can be sent from plugins to X-Plane to allocate and release memory.

You will need to modify LuaJIT to allow the custom allocator API on 64-bit builds. Contact Ben if you need a pre-compiled static library that has the custom allocator API re-enabled.

You can use the dataref sim/operation/prefs/misc/has_lua_alloc (type int, read only) to determine whether the custom allocator message API is available. If it is, you _must_ use it.

Sample Code to Use X-Plane’s LuaJIT Allocator

/* Include XPLM headers, etc. */

struct lua_alloc_request_t {
			void *	ud;
			void *	ptr;
			size_t	osize;
			size_t	nsize;
};

#define		ALLOC_OPEN		0x00A110C1
#define		ALLOC_REALLOC	0x00A110C2
#define		ALLOC_CLOSE		0x00A110C3
/* new in X-Plane 10.40 */
#define		ALLOC_LOCK		0x00A110C4
#define		ALLOC_UNLOCK		0x00A110C5
#define		ALLOC_LOCK_RO		0x00A110C6

/* lua interpreter */
lua_State* l;

static void *lj_alloc_create(void)
{
	struct lua_alloc_request_t r = { 0 };
	XPLMSendMessageToPlugin(XPLM_PLUGIN_XPLANE, ALLOC_OPEN,&r);
	return r.ud;	
}

static void  lj_alloc_destroy(void *msp)
{
	struct lua_alloc_request_t r = { 0 };
	r.ud = msp;
	XPLMSendMessageToPlugin(XPLM_PLUGIN_XPLANE, ALLOC_CLOSE,&r);
}

static void *lj_alloc_f(void *msp, void *ptr, size_t osize, size_t nsize)
{
	struct lua_alloc_request_t r = { 0 };
	r.ud = msp;
	r.ptr = ptr;
	r.osize = osize;
	r.nsize = nsize;
	XPLMSendMessageToPlugin(XPLM_PLUGIN_XPLANE, ALLOC_REALLOC,&r);
	return r.ptr;
}

void * ud;
lua_State * l;

/* when you need to init LUA... */

  XPLMDataRef lua_alloc_ref = XPLMFindDataRef("sim/operation/prefs/misc/has_lua_alloc");
  if(lua_alloc_ref && XPLMGetDatai(lua_alloc_ref))
  {
    /* X-Plane has an allocator for us - we _must_ use it. */
    ud = lj_alloc_create();
    if (ud == NULL)
    {
       /* handle error */
    }
    l = lua_newstate(lj_alloc_f, ud);
  }
  else
  {
    /* use the default allocator. */
    l = luaL_newstate();
  }

/* when you need to clean up lua */

  XPLMDataRef lua_alloc_ref = XPLMFindDataRef("sim/operation/prefs/misc/has_lua_alloc");
  if(lua_alloc_ref && XPLMGetDatai(lua_alloc_ref))
  {
    lua_close(l);
    lj_alloc_destroy(ud);
  }
  else
  {
    lua_close(l);
  }

Test Materials for LuaJIT 2.0.1 64-bit

The official version of LuaJIT does not allow custom allocators in 64-bit builds. (They did this to prevent people from accidentally allocating high memory.) To wire LuaJIT to X-Plane’s allocator (which does provide low memory as needed), a modified build of LuaJIT is needed.

Here is a link to the source for LuaJIT with the custom 64-bit allocator re-enabled:

Windows builds are compiled with MSVC2010; the runtime has been set to /MT to avoid a DLL dependency on the MSVC runtime (which is not required for X-Plane itself!) Mac builds are compiled on OS X 10.6 and Linux on Ubuntu 10.04.

To test LuaJIT allocation, get X-Plane 10.22 beta 1 or newer; 10.22 contains 64-bit LuaJIT allocation support for all three operating systems.

Viewing Lua Memory Usage

You can show you current total bytes allocated by the lua-allocation-service:

To view the allocations, use DataRefEditor – pick “view stats” and then filter for “lua”. You will see the bytes go up and then back down when garbage collection kicks in; if the bytes go up continuously, you may have a leak!

Memory Protection for Lua

Starting with X-Plane 10.40, you can request that X-Plane temporarily lock access to the memory allocated for your plugin for Lua use. Two new messages (ALLOC_LOCK = 0xA110C4 and ALLOC_UNLOCK = 0xA110C5) make the change, e.g.

void lock_lua_memory()
{
    XPLMSendMessageToPlugin(XPLM_PLUGIN_XPLANE, ALLOC_LOCK, NULL);
}

void unlock_lua_memory()
{
    XPLMSendMessageToPlugin(XPLM_PLUGIN_XPLANE, ALLOC_UNLOCK, NULL);
}

void lock_read_only_lua_memory()
{
    XPLMSendMessageToPlugin(XPLM_PLUGIN_XPLANE, ALLOC_LOCK_RO, NULL);
}

Your memory starts unlocked; when you send a lock message, the underlying VM pages allocated for use by Lua are set to not be readable or writable; any code that then accesses them will crash x-plane at the point of access.

The purpose of this lock is to detect memory corruption immediately; while the lock is on, code that is corrupting lua memory will crash with a back-trace pointing to the offending code.

Note that the lock:

  • Must be off for you to run any lua API calls, including executing Lua code, reading from the interpreter, or creating/destroying Lua contexts.
  • Locks/unlocks only your plugins memory.

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