[PXE] Add PMM support to romprefix.S (untested)

ROM initialisation vector now attempts to allocate a 2MB block using
PMM.  If successful, it copies the ROM image to this block, then
shrinks the ROM image to allow for more option ROMs.  If unsuccessful,
it leaves the ROM as-is.

ROM BEV now attempts to return to the BIOS, resorting to INT 18 only
if the BIOS stack has been corrupted.

src/arch/i386/prefix/libprefix.S

@@ -17,6 +17,10 @@
  *
  */
 
+	.arch i386
+	.section ".prefix.lib", "awx", @progbits
+	.section ".data16", "aw", @progbits
+
 /**
  * High memory temporary load address
  *
@@ -27,25 +31,18 @@
  * We use the start of an even megabyte so that we don't have to worry
  * about the current state of the A20 line.
  *
- * We use 4MB rather than 2MB because there is at least one commercial
- * PXE ROM ("Broadcom UNDI, PXE-2.1 (build 082) v2.0.4") which stores
- * data required by the UNDI ROM loader (yes, the ROM loader; that's
- * the component which should be impossible to damage short of
- * screwing with the MMU) around the 2MB mark.  Sadly, this is not a
- * joke.
- *
+ * We use 4MB rather than 2MB because some PXE stack / PMM BIOS
+ * combinations are known to place data required by other UNDI ROMs
+ * loader around the 2MB mark.
  */
-#define HIGHMEM_LOADPOINT ( 4 << 20 )
+	.globl	HIGHMEM_LOADPOINT
+	.equ	HIGHMEM_LOADPOINT, ( 4 << 20 )
 
 /* Image compression enabled */
 #define COMPRESS 1
 
 #define CR0_PE 1
 
-	.arch i386
-	.section ".prefix.lib", "awx", @progbits
-	.section ".data16", "aw", @progbits
-
 /****************************************************************************
  * pm_call (real-mode near call)
  *
@@ -223,12 +220,15 @@ copy_bytes:
  * Returns:
  *   %ds:esi : next source address (will be a multiple of 16)
  * Corrupts:
- *   %edi, %ecx, %edx
+ *   %ecx, %edx
  ****************************************************************************
  */
 	.section ".prefix.lib"
 	.code16
 install_block:
+	/* Preserve registers */
+	pushl	%edi
+
 #if COMPRESS
 	/* Decompress source to destination */
 	call	decompress16
@@ -249,6 +249,8 @@ install_block:
 	addl	$0xf, %esi
 	andl	$~0xf, %esi
 
+	/* Restore registers and return */
+	popl	%edi
 	ret
 	.size install_block, . - install_block
 
@@ -270,6 +272,7 @@ install_block:
  */
 	.section ".prefix.lib"
 	.code16
+	.globl	alloc_basemem
 alloc_basemem:
 	/* FBMS => %ax as segment address */
 	movw	$0x40, %ax
@@ -308,13 +311,14 @@ alloc_basemem:
  * Returns:
  *   %esi : next source physical address (will be a multiple of 16)
  * Corrupts:
- *   %edi, %ecx, %edx
+ *   %ecx, %edx
  ****************************************************************************
  */
 	.section ".prefix.lib"
 	.code16
 install_basemem:
 	/* Preserve registers */
+	pushl	%edi
 	pushw	%ds
 
 	/* Preserve original %esi */
@@ -335,6 +339,7 @@ install_basemem:
 
 	/* Restore registers */
 	popw	%ds
+	popl	%edi
 	ret
 	.size install_basemem, . - install_basemem
 
@@ -351,7 +356,7 @@ install_basemem:
  * Returns:
  *   %esi : next source physical address (will be a multiple of 16)
  * Corrupts:
- *   %edi, %ecx, %edx
+ *   %ecx, %edx
  ****************************************************************************
  */
 
@@ -376,17 +381,14 @@ install_highmem:
 
 /****************************************************************************
  * install (real-mode near call)
- * install_prealloc (real-mode near call)
  *
  * Install all text and data segments.
  *
  * Parameters:
- *   %ax : .text16 segment address (install_prealloc only)
- *   %bx : .data16 segment address (install_prealloc only)
+ *   none
  * Returns:
- *   %ax : .text16 segment address
- *   %bx : .data16 segment address
- *   %edi : .text physical address (if applicable)
+ *   %ax  : .text16 segment address
+ *   %bx  : .data16 segment address
  * Corrupts:
  *   none
  ****************************************************************************
@@ -395,26 +397,55 @@ install_highmem:
 	.code16
 	.globl install
 install:
+	/* Preserve registers */
+	pushl	%esi
+	pushl	%edi
 	/* Allocate space for .text16 and .data16 */
 	call	alloc_basemem
+	/* Image source = %cs:0000 */
+	xorl	%esi, %esi
+	/* Image destination = HIGHMEM_LOADPOINT */
+	movl	$HIGHMEM_LOADPOINT, %edi
+	/* Install text and data segments */
+	call	install_prealloc
+	/* Restore registers and return */
+	popl	%edi
+	popl	%esi
+	ret
 	.size install, . - install
+
+/****************************************************************************
+ * install_prealloc (real-mode near call)
+ *
+ * Install all text and data segments.
+ *
+ * Parameters:
+ *   %ax  : .text16 segment address
+ *   %bx  : .data16 segment address
+ *   %esi : Image source physical address (or zero for %cs:0000)
+ *   %edi : Decompression temporary area physical address
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+	.section ".prefix.lib"
+	.code16
 	.globl install_prealloc
 install_prealloc:
 	/* Save registers */
+	pushal
 	pushw	%ds
 	pushw	%es
-	pushl	%esi
-	pushl	%ecx
-	pushl	%edx
 
 	/* Sanity: clear the direction flag asap */
 	cld
 
 	/* Calculate physical address of payload (i.e. first source) */
-	xorl	%esi, %esi
+	testl	%esi, %esi
+	jnz	1f
 	movw	%cs, %si
 	shll	$4, %esi
-	addl	$_payload_offset, %esi
+1:	addl	$_payload_offset, %esi
 
 	/* Install .text16 */
 	movw	%ax, %es
@@ -440,7 +471,6 @@ install_prealloc:
 	 * prior to reading the E820 memory map and relocating
 	 * properly.
 	 */
-	movl	$HIGHMEM_LOADPOINT, %edi
 	movl	$_textdata_progbits_size, %ecx
 	movl	$_textdata_size, %edx
 	pushl	%edi
@@ -473,11 +503,9 @@ install_prealloc:
 
 #endif
 	/* Restore registers */
-	popl	%edx
-	popl	%ecx
-	popl	%esi
 	popw	%es
 	popw	%ds
+	popal
 	ret
 	.size install_prealloc, . - install_prealloc