Define zero-page offset 0x1e4 as a scratch field, and use it

The relocatable kernel code needs a scratch field for the decompressor
to determine its own location.  It was using a location inside
struct screen_info; reserve a free location and document it as scratch
instead.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 7 insertions, 6 deletions

diff --git a/Documentation/i386/zero-page.txt b/Documentation/i386/zero-page.txt
index c04a421f4a7c..75b3680c41eb 100644
--- a/Documentation/i386/zero-page.txt
+++ b/Documentation/i386/zero-page.txt
@@ -37,6 +37,7 @@ Offset	Type		Description
 0x1d0   unsigned long   EFI memory descriptor map pointer
 0x1d4   unsigned long   EFI memory descriptor map size
 0x1e0   unsigned long   ALT_MEM_K, alternative mem check, in Kb
+0x1e4   unsigned long   Scratch field for the kernel setup code
 0x1e8   char            number of entries in E820MAP (below)
 0x1e9   unsigned char   number of entries in EDDBUF (below)
 0x1ea   unsigned char   number of entries in EDD_MBR_SIG_BUFFER (below)
diff --git a/arch/i386/boot/compressed/head.S b/arch/i386/boot/compressed/head.S
index 3517a32aaf41..f35ea2237522 100644
--- a/arch/i386/boot/compressed/head.S
+++ b/arch/i386/boot/compressed/head.S
@@ -45,10 +45,10 @@ startup_32:
  * at and where we were actually loaded at.  This can only be done
  * with a short local call on x86.  Nothing  else will tell us what
  * address we are running at.  The reserved chunk of the real-mode
- * data at 0x34-0x3f are used as the stack for this calculation.
- * Only 4 bytes are needed.
+ * data at 0x1e4 (defined as a scratch field) are used as the stack
+ * for this calculation. Only 4 bytes are needed.
  */
-        leal 0x40(%esi), %esp
+        leal (0x1e4+4)(%esi), %esp
         call 1f
 1:      popl %ebp
         subl $1b, %ebp
diff --git a/arch/x86_64/boot/compressed/head.S b/arch/x86_64/boot/compressed/head.S
index f9d5692a0106..1312bfaff306 100644
--- a/arch/x86_64/boot/compressed/head.S
+++ b/arch/x86_64/boot/compressed/head.S
@@ -46,10 +46,10 @@ startup_32:
  * at and where we were actually loaded at.  This can only be done
  * with a short local call on x86.  Nothing  else will tell us what
  * address we are running at.  The reserved chunk of the real-mode
- * data at 0x34-0x3f are used as the stack for this calculation.
- * Only 4 bytes are needed.
+ * data at 0x1e4 (defined as a scratch field) are used as the stack
+ * for this calculation. Only 4 bytes are needed.
  */
-        leal    0x40(%esi), %esp
+        leal    (0x1e4+4)(%esi), %esp
         call    1f
 1:      popl    %ebp
         subl    $1b, %ebp