[pxe] Introduce PXE exit hook for NBP chaining

It might be the case that we wish to chain to an NBP without
being "in the way".  We now implement a hook in our exit path
for gPXE *.*pxe build targets.  The hook is a pointer to a
SEG16:OFF16 which we try to jump to during exit.  By default,
this pointer results in the usual exit path.

We also implement the "pxenv_file_exit_hook" PXE API routine
to allow the user to specify an alternate SEG16:OFF16 to jump
to during exit.

Unfortunately, this additional PXE extension has a cost
in code size.  Fortunately, a look at the size difference
for a gPXE .rom build target shows zero size difference
after compression.

The routine is documented in doc/pxe_extensions as follows:

FILE EXIT HOOK

Op-Code:	PXENV_FILE_EXIT_HOOK (00e7h)

Input:	Far pointer to a t_PXENV_FILE_EXIT_HOOK parameter
		structure that has been initialized by the caller.

Output:	PXENV_EXIT_SUCCESS or PXENV_EXIT_FAILURE must be
		returned in AX.  The Status field in the parameter
		structure must be set to one of the values represented
		by the PXENV_STATUS_xxx constants.

Description:Modify the exit path to jump to the specified code.
		Only valid for pxeprefix-based builds.

typedef struct s_PXENV_FILE_EXIT_HOOK {
        PXENV_STATUS_t Status;
        SEGOFF16_t Hook;
} t_PXENV_FILE_EXIT_HOOK;

Set before calling API service:

Hook:	The SEG16:OFF16 of the code to jump to.

Returned from API service:

Status:	See PXENV_STATUS_xxx constants.

Requested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Shao Miller <shao.miller@yrdsb.edu.on.ca>
Signed-off-by: Marty Connor <mdc@etherboot.org>

src/arch/i386/include/pxe.h

 	struct s_PXENV_GET_FILE_SIZE		get_file_size;
 	struct s_PXENV_FILE_EXEC		file_exec;
 	struct s_PXENV_FILE_API_CHECK		file_api_check;
+	struct s_PXENV_FILE_EXIT_HOOK		file_exit_hook;
 };
 
 typedef union u_PXENV_ANY PXENV_ANY_t;

src/arch/i386/include/pxe_api.h

 
 /** @} */ /* pxenv_file_api_check */
 
+/** @defgroup pxenv_file_exit_hook PXENV_FILE_EXIT_HOOK
+ *
+ * FILE EXIT HOOK
+ *
+ * @{
+ */
+
+/** PXE API function code for pxenv_file_exit_hook() */
+#define PXENV_FILE_EXIT_HOOK			0x00e7
+
+/** Parameter block for pxenv_file_exit_hook() */
+struct s_PXENV_FILE_EXIT_HOOK {
+	PXENV_STATUS_t Status;		/**< PXE status code */
+	SEGOFF16_t Hook;		/**< SEG16:OFF16 to jump to */
+} PACKED;
+
+typedef struct s_PXENV_FILE_EXIT_HOOK PXENV_FILE_EXIT_HOOK_t;
+
+extern PXENV_EXIT_t pxenv_file_exit_hook ( struct s_PXENV_FILE_EXIT_HOOK *file_exit_hook );
+
+/** @} */ /* pxenv_file_exit_hook */
+
 /** @} */ /* pxe_file_api */
 
 /** @defgroup pxe_loader_api PXE Loader API

src/arch/i386/interface/pxe/pxe_call.c

 	PXENV_EXIT_t ( * get_file_size ) ( struct s_PXENV_GET_FILE_SIZE * );
 	PXENV_EXIT_t ( * file_exec ) ( struct s_PXENV_FILE_EXEC * );
 	PXENV_EXIT_t ( * file_api_check ) ( struct s_PXENV_FILE_API_CHECK * );
+	PXENV_EXIT_t ( * file_exit_hook ) ( struct s_PXENV_FILE_EXIT_HOOK * );
 };
 
 /**
 		pxenv_call.file_api_check = pxenv_file_api_check;
 		param_len = sizeof ( pxenv_any.file_api_check );
 		break;
+	case PXENV_FILE_EXIT_HOOK:
+		pxenv_call.file_exit_hook = pxenv_file_exit_hook;
+		param_len = sizeof ( pxenv_any.file_exit_hook );
+		break;
 	default:
 		DBG ( "PXENV_UNKNOWN_%hx", opcode );
 		pxenv_call.unknown = pxenv_unknown;

src/arch/i386/interface/pxe/pxe_file.c

 #include <gpxe/posix_io.h>
 #include <gpxe/features.h>
 #include <pxe.h>
+#include <realmode.h>
 
 /*
  * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
+ * Portions (C) 2010 Shao Miller <shao.miller@yrdsb.edu.on.ca>.
+ *              [PXE exit hook logic]
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
 	return PXENV_EXIT_SUCCESS;
 }
 
+segoff_t __data16 ( pxe_exit_hook ) = { 0, 0 };
+#define pxe_exit_hook __use_data16 ( pxe_exit_hook )
+
 /**
  * FILE API CHECK
  *
 		file_api_check->Magic    = 0xe9c17b20;
 		file_api_check->Provider = 0x45585067; /* "gPXE" */
 		file_api_check->APIMask  = 0x0000007f; /* Functions e0-e6 */
+		/* Check to see if we have a PXE exit hook */
+		if ( pxe_exit_hook.segment | pxe_exit_hook.offset )
+			/* Function e7, also */
+			file_api_check->APIMask |= 0x00000080;
 		file_api_check->Flags    = 0;	       /* None defined */
 		return PXENV_EXIT_SUCCESS;
 	}
 }
+
+/**
+ * FILE EXIT HOOK
+ *
+ * @v file_exit_hook			Pointer to a struct
+ *					s_PXENV_FILE_EXIT_HOOK
+ * @v s_PXENV_FILE_EXIT_HOOK::Hook	SEG16:OFF16 to jump to
+ * @ret #PXENV_EXIT_SUCCESS		Successfully set hook
+ * @ret #PXENV_EXIT_FAILURE		We're not an NBP build
+ * @ret s_PXENV_FILE_EXIT_HOOK::Status	PXE status code
+ *
+ */
+PXENV_EXIT_t pxenv_file_exit_hook ( struct s_PXENV_FILE_EXIT_HOOK
+					*file_exit_hook ) {
+	DBG ( "PXENV_FILE_EXIT_HOOK" );
+
+	/* Check to see if we have a PXE exit hook */
+	if ( pxe_exit_hook.segment | pxe_exit_hook.offset ) {
+		/* We'll jump to the specified SEG16:OFF16 during exit */
+		pxe_exit_hook.segment = file_exit_hook->Hook.segment;
+		pxe_exit_hook.offset = file_exit_hook->Hook.offset;
+		file_exit_hook->Status = PXENV_STATUS_SUCCESS;
+		return PXENV_EXIT_SUCCESS;
+	}
+
+	DBG ( " not NBP" );
+	file_exit_hook->Status = PXENV_STATUS_UNSUPPORTED;
+	return PXENV_EXIT_FAILURE;
+}
+

src/arch/i386/prefix/pxeprefix.S

 	lret
 	.section ".text16", "ax", @progbits
 1:
+	/* Update the exit hook */
+	movw	%cs,pxe_exit_hook+2
+	push	%ax
+	mov	$2f,%ax
+	mov	%ax,pxe_exit_hook
+	pop	%ax
+
 	/* Run main program */
 	pushl	$main
 	pushw	%cs
 	movw	%di, %ss
 	movl	%ebp, %esp
 
-	/* Check PXE stack magic */
+	/* Jump to hook if applicable */
+	ljmpw	*pxe_exit_hook
+
+2:	/* Check PXE stack magic */
 	popl	%eax
 	cmpl	$STACK_MAGIC, %eax
 	jne	1f

src/doc/pxe_extensions

 APIMask:	Bitmask of supported API functions (one bit for each function
 		in the range 00e0h to 00ffh).
 Flags:		Set to zero, reserved for future use.
+
+
+
+
+FILE EXIT HOOK
+
+Op-Code:	PXENV_FILE_EXIT_HOOK (00e7h)
+
+Input:		Far pointer to a t_PXENV_FILE_EXIT_HOOK parameter
+		structure that has been initialized by the caller.
+
+Output:		PXENV_EXIT_SUCCESS or PXENV_EXIT_FAILURE must be
+		returned in AX.  The Status field in the parameter
+		structure must be set to one of the values represented
+		by the PXENV_STATUS_xxx constants.
+
+Description:	Modify the exit path to jump to the specified code.
+		Only valid for pxeprefix-based builds.
+
+typedef struct s_PXENV_FILE_EXIT_HOOK {
+        PXENV_STATUS_t Status;
+        SEGOFF16_t Hook;
+} t_PXENV_FILE_EXIT_HOOK;
+
+
+Set before calling API service:
+
+Hook:		The SEG16:OFF16 of the code to jump to.
+
+
+Returned from API service:
+
+Status:		See PXENV_STATUS_xxx constants.