[romprefix] Inhibit the use of relocation during POST

It is common for system memory maps to be grotesquely unreliable
during POST.  Many sanity checks have been added to the memory map
reading code, but these do not catch all problems.

Skip relocation entirely if called during POST.  This should avoid the
problems typically encountered, at the cost of slightly disrupting the
memory map of an operating system booted via iPXE when iPXE was
entered during POST.  Since this is a very rare special case (used,
for example, when reflashing an experimental ROM that would otherwise
prevent the system from completing POST), this is an acceptable cost.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/libprefix.S

@@ -443,6 +443,8 @@ install:
 	xorl	%esi, %esi
 	/* Image destination = default */
 	xorl	%edi, %edi
+	/* Allow relocation */
+	clc
 	/* Install text and data segments */
 	call	install_prealloc
 	/* Restore registers and return */
@@ -461,6 +463,7 @@ install:
  *   %bx  : .data16 segment address
  *   %esi : Image source physical address (or zero for %cs:0000)
  *   %edi : Decompression temporary area physical address (or zero for default)
+ *   CF set : Avoid relocating to top of memory
  * Corrupts:
  *   none
  ****************************************************************************
@@ -473,9 +476,8 @@ install_prealloc:
 	pushal
 	pushw	%ds
 	pushw	%es
-
-	/* Sanity: clear the direction flag asap */
-	cld
+	cld			/* Sanity: clear the direction flag asap */
+	pushfw
 
 	/* Copy decompression temporary area physical address to %ebp */
 	movl	%edi, %ebp
@@ -570,6 +572,11 @@ a20_death_message:
 	movl	%ebp, %edi
 	lcall	*init_librm_vector
 
+	/* Skip relocation if CF was set on entry */
+	popfw
+	pushfw
+	jc	skip_relocate
+
 	/* Call relocate() to determine target address for relocation.
 	 * relocate() will return with %esi, %edi and %ecx set up
 	 * ready for the copy to the new location.
@@ -588,9 +595,11 @@ a20_death_message:
 
 	/* Initialise librm at new location */
 	lcall	*init_librm_vector
+skip_relocate:
 #endif
 
 	/* Restore registers */
+	popfw
 	popw	%es
 	popw	%ds
 	popal