[comboot] Allow for tail recursion of COMBOOT images

Multi-level menus via COMBOOT rely on the COMBOOT program being able
to exit and invoke a new COMBOOT program (the next menu).  This works,
but rapidly (within about five iterations) runs out of space in gPXE's
internal stack, since each new image is executed in a new function
context.

Fix by allowing tail recursion between images; an image can now
specify a replacement image for itself, and image_exec() will perform
the necessary tail recursion.

1 files changed, 5 insertions, 9 deletions

diff --git a/src/arch/i386/include/comboot.h b/src/arch/i386/include/comboot.h
index 1fc3b718b..4376650f6 100644
--- a/src/arch/i386/include/comboot.h
+++ b/src/arch/i386/include/comboot.h
@@ -79,18 +79,14 @@ extern int comboot_resolv ( const char *name, struct in_addr *address );
 /* setjmp/longjmp context buffer used to return after loading an image */
 extern jmp_buf comboot_return;
 
-/* Command line to execute when returning via comboot_return 
- * with COMBOOT_RETURN_RUN_KERNEL
- */
-extern char *comboot_kernel_cmdline;
-
-/* Execute comboot_image_cmdline */
-extern void comboot_run_kernel ( );
+/* Replacement image when exiting with COMBOOT_EXIT_RUN_KERNEL */
+extern struct image *comboot_replacement_image;
 
 extern void *com32_external_esp;
 
-#define COMBOOT_RETURN_EXIT 1
-#define COMBOOT_RETURN_RUN_KERNEL 2
+#define COMBOOT_EXIT 1
+#define COMBOOT_EXIT_RUN_KERNEL 2
+#define COMBOOT_EXIT_COMMAND 3
 
 extern void comboot_force_text_mode ( void );