Re: [Xen-devel] [PATCH 3/7] vgabios: Fix size computation overflow

[Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>]

On Fri, 2012-05-04 at 15:26 +0100, Ian Campbell wrote:
> On Fri, 2012-05-04 at 15:18 +0100, Frediano Ziglio wrote:
> > On Fri, 2012-05-04 at 15:12 +0100, Ian Campbell wrote:
> > > On Fri, 2012-05-04 at 13:36 +0100, Frediano Ziglio wrote:
> > > > Remove an overflow computing width x height x bit which does
> > > > not fit into a 16 bits. I wrote a routine to multiple these value
> > > > and get the size required for framebuffer in segment unit (64k).
> > > 
> > > Couldn't this be done in C using a suitably wide temporary variable?
> > > 
> > 
> > I'd like :(
> > 
> > BCC compiler used need some function which are not linked by BIOS.
> 
> In which case 
> 
> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> 
> 

In current vgabios (from Bochs not Xen copy) size is computed using 


size_64k = (Bit16u)((Bit32u)cur_info->info.XResolution * 
cur_info->info.XResolution * cur_info->info.BitsPerPixel) >> 19;


If you try to use this code you get an undefined call to lmulul. Bochs
code (still in vbe.c) provide this internal function with this asm code:


; helper function for memory size calculation

lmulul:
  and eax, #0x0000FFFF
  shl ebx, #16
  or  eax, ebx
  SEG SS
  mul eax, dword ptr [di]
  mov ebx, eax
  shr ebx, #16
  ret


Note also that Bochs line have some problems
- it multiply Xres by Xres intead of Xres by Yref
- it does not take into account ceiling division

Frediano


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