Re: [Xen-devel] [PATCH][QEMU] Clear bios framebuffer with minimal writes

[Gary Grebus <ggrebus@xxxxxxxxxxxxxxx>]

On Thu, 2007-10-25 at 15:20 +0100, Keir Fraser wrote:
> On 24/10/07 22:04, "Ben Guthro" <bguthro@xxxxxxxxxxxxxxx> wrote:
> 
> > Support for VGA bios to clear framebuffer with minimal writes.
> > 
> > Signed-off-by: Ben Guthro <bguthro@xxxxxxxxxxxxxx>
> > Signed-off-by: Gary Grebus <ggrebus@xxxxxxxxxxxxxxx>
> 
> Is this really just a workaround for the insane-sounding case of Vista
> clearing the screen, as described in one of the comments? Do I understand
> correctly, and it looks like Vista does 4096 writes for each iteration of
> the rep stosw?

Yep.  This happens when resizing the screen in Vista.  It appears to do
this by executing the VGA BIOS code in some sort of emulation, and the
BIOS function to clear the screen (which uses a rep stosw) gets emulated
as 4096 separate writes.

This is so slow that occasionally some watchdog timer in Vista (that was
timing the BIOS emulation) would expire and cause a blue screen.

This fix was a bit of a hack.  It might be possible to speed up VGA
writes enough to make this unnecessary, but that was more of a challenge
than I wanted at the time. 

        /gary

-- 
Gary Grebus
Virtual Iron Software, Inc.



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