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Date: Tue, 24 Mar 2009 03:48:29 +0300 (MSK)
From: malc <av1474@comtv.ru>
Subject: Re: [Qemu-devel] [PATCH] compute checksum for roms bigger than a
	segment
In-Reply-To: <1237850625-15864-1-git-send-email-glommer@redhat.com>
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To: qemu-devel@nongnu.org

On Mon, 23 Mar 2009, Glauber Costa wrote:

> 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 <glommer@redhat.com>
> ---
>  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

     xchg dx, cx
     dec  cx

> +
>  checksum_loop:
>    add  al, [bx]
>    inc  bx
>    loop checksum_loop
> +
> +  cmp  dx, #0

     test dx, dx

> +  je checksum_out
> +
> +  add  al, [bx]

     DS:[0] is added twice, intended?

> +  mov  cx, dx
> +  mov  dx, ds
> +  add  dx, #0x1000

     add  dh, #0x10

> +  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
>  
>  
> 

-- 
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