Re: [Xen-devel] [Qemu-devel] Cirrus VGA slow screen update, show blank screen last 13s or so for windows XP guest

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

Windows XP uses VGA planar mode during boot up.   This means that to
set a pixel requires 3 MMIO operations (one for the red pixel, one for
the green pixel, one for the blue pixel).  KVM has an optimization
called MMIO coalescing which avoids a heavy weight exit for planar
mode exits.  I guess Xen doesn't have an optimization like this.

Regards,

Anthony Liguori

On Thu, Jul 25, 2013 at 8:13 AM, Gonglei (Arei) <arei.gonglei@xxxxxxxxxx> wrote:
> Hi,
>   I found a problem: For windows XP guest booting by qemu upstream,
> using the RDP(Remote Desktop Protocol) and VNC protocol to connect the 
> windows XP guest
> which booting by Qemu upstream respectively. the VNC client will become blank 
> screen last
> 13 seconds or so on XEN platform, and last 2 seconds on KVM. And through 
> debugging,
> the cirrus VGA card does not produce dirty pages during the blank screen's 
> times.
>
>   When -vga is changed from cirrus to std the issue goes away and the update 
> proceeds at normal speed.
> In addition, when using the Qemu-dm replace Qemu upstream on XEN platform,
> the cirrus VGA works good and the issue goes away also.
>
>   Today, lots of googling have seen the same behavior:
>
> https://github.com/joyent/smartos-live/issues/215
> http://lists.gnu.org/archive/html/qemu-devel/2011-06/msg02105.html
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=574988
> etc...
>
>   But, I test the latest qemu-1.5.1, the problem exist firmly.
>
>   Any ideas? Thanks!
>
> Best Regards!
> -Gonglei

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