[Qemu-devel] [PATCH] compute checksum for roms bigger than a segment

[Glauber Costa]

Some option roms (e1000 provided by gpxe project as an example)
are bigger than a segment. The current algorithm to compute the
checksum fails in such case. To proper compute the checksum, this
patch deals with the possibility of the rom's size crossing a
segment border.

We don't need to worry about it crossing more than one segment
border, since the option roms format only save one byte to store
the image size (thus, maximum size = 0xff = 128k = 2 segments)

Signed-off-by: Glauber Costa <address@hidden>
---
 bios/rombios.c |   33 +++++++++++++++++++++++++++------
 1 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/bios/rombios.c b/bios/rombios.c
index bc43251..6fede17 100644
--- a/bios/rombios.c
+++ b/bios/rombios.c
@@ -10162,22 +10162,43 @@ no_serial:
   ret
 
 rom_checksum:
-  push ax
-  push bx
-  push cx
+  pusha
+  push ds
+
   xor  ax, ax
   xor  bx, bx
   xor  cx, cx
+  xor  dx, dx
+
   mov  ch, [2]
   shl  cx, #1
+
+  jnc checksum_loop
+  mov  dx, cx
+  mov  cx, #0xffff
+
 checksum_loop:
   add  al, [bx]
   inc  bx
   loop checksum_loop
+
+  cmp  dx, #0
+  je checksum_out
+
+  add  al, [bx]
+  mov  cx, dx
+  mov  dx, ds
+  add  dx, #0x1000
+  mov  ds, dx
+  xor  dx, dx 
+  xor  bx, bx
+
+  jmp  checksum_loop
+
+checksum_out:
   and  al, #0xff
-  pop  cx
-  pop  bx
-  pop  ax
+  pop  ds
+  popa 
   ret
 
 
-- 
1.6.1.2