Xen devel - Oct 2007 - [PATCH][QEMU] Clear bios framebuffer with minimal writes

Support for VGA bios to clear framebuffer with minimal writes.

Signed-off-by: Ben Guthro <bguthro@virtualron.com>
Signed-off-by: Gary Grebus <ggrebus@virtualiron.com>



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On 24/10/07 22:04, "Ben Guthro" <bguthro@virtualiron.com> wrote:
> Support for VGA bios to clear framebuffer with minimal writes.
> 
> Signed-off-by: Ben Guthro <bguthro@virtualron.com>
> Signed-off-by: Gary Grebus <ggrebus@virtualiron.com>
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?

 -- Keir



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On Thu, 2007-10-25 at 15:20 +0100, Keir Fraser wrote:
> On 24/10/07 22:04, "Ben Guthro" <bguthro@virtualiron.com>
wrote:
> 
> > Support for VGA bios to clear framebuffer with minimal writes.
> > 
> > Signed-off-by: Ben Guthro <bguthro@virtualron.com>
> > Signed-off-by: Gary Grebus <ggrebus@virtualiron.com>
> 
> 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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On 25/10/07 16:23, "Gary Grebus" <ggrebus@virtualiron.com>
wrote:
>> 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.
Ugh, that is awkward. Did you do this hack before the stdvga
emulation-in-xen and buffered-io patch that you guys also sent out today? I
wonder whether that speeds up vga writes enough, or whether you needed this
hack despite the speedups that other patch achieved? Overall, I guess
I''m
not against this patch even though it is a bit sleazy. :-) But obviously
it''s even better if the problem goes away by other means...

 -- Keir



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On Thu, 2007-10-25 at 16:34 +0100, Keir Fraser wrote:
> Ugh, that is awkward. Did you do this hack before the stdvga
> emulation-in-xen and buffered-io patch that you guys also sent out today? I
> wonder whether that speeds up vga writes enough, or whether you needed this
> hack despite the speedups that other patch achieved? Overall, I guess
I''m
> not against this patch even though it is a bit sleazy. :-) But obviously
> it''s even better if the problem goes away by other means...
> 
This patch predates the stdvga emulation-in-xen and buffered-io changes.
I''ll try with only the later changes and see if they are sufficient.

	/gary

-- 
Gary Grebus
Virtual Iron Software, Inc.



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On Thu, 2007-10-25 at 13:14 -0400, Gary Grebus wrote:
> On Thu, 2007-10-25 at 16:34 +0100, Keir Fraser wrote:
> 
> > Ugh, that is awkward. Did you do this hack before the stdvga
> > emulation-in-xen and buffered-io patch that you guys also sent out
today? I
> > wonder whether that speeds up vga writes enough, or whether you needed
this
> > hack despite the speedups that other patch achieved? Overall, I guess
I''m
> > not against this patch even though it is a bit sleazy. :-) But
obviously
> > it''s even better if the problem goes away by other means...
> > 
> 
> This patch predates the stdvga emulation-in-xen and buffered-io changes.
> I''ll try with only the later changes and see if they are
sufficient.
> 
I tried with only the stdvga and buffered-io patch.  Things got worse,
to the point that Vista no longer boots, encountering a BSOD due to a
timeout in Vista graphics driver.

Apparently the stdvga and buffered-io patch de-optimizes this particular
case, but we didn''t notice due to the vgabios patch.

The vgabios patch fixes the intermittent "BSOD on screen resize"
problem
either way.  We won''t know if it''s really necessary until we
can fix the
stdvga etc. patch which at the moment looks a little suspect.

	/gary




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