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>]

 On Thu, Jul 25, 2013 at 10:08 PM, Gonglei (Arei)
<arei.gonglei@xxxxxxxxxx> wrote:
>> -----Original Message-----
>> From: Anthony Liguori [mailto:anthony@xxxxxxxxxxxxx]
>> Sent: Friday, July 26, 2013 10:09 AM
>> To: Gonglei (Arei)
>> Cc: qemu-devel@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; Hanweidong;
>> Luonengjun; Huangweidong (Hardware)
>> Subject: Re: [Qemu-devel] Cirrus VGA slow screen update, show blank screen
>> last 13s or so for windows XP guest
>>
>> 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
>
> Thanks, Anthony.
> I tested the Windows XP guest again on KVM and XEN, and I found:
> 1. Windows XP will show blank screen during boot up as the same as
>  the protocol switch between VNC and RDP.
> 2. Standard VGA works well, what's difference between cirrus and standard VGA 
> for the guest?

Cirrus predates VESA.  VESA has a linear framebuffer mode and WinXP
can use VESA modes (provided that VESA drivers are installed).  A
linear framebuffer mode requires no exits to update individual pixels.
 Instead, there is a timer that fires 30-60 times a second to handle
any pixels that have been dirtied during that period.

> 3. Why does the traditional qemu has no blank screen problem on cirrus VGA 
> emulation ?

This is one of the few cases where TCG is actually faster than KVM or
Xen.  In TCG, an MMIO exit is converted to a function call.  OTOH,
even with KVM, an MMIO exit is at least a couples thousand cycles.
It's worse with Xen because dom0 has to be scheduled.

If you search a bit for the V2E project, there was an attempt to
combine TCG emulation with hardware virtualization specifically to
handle cases like this.  Coalesced MMIO was good enough for KVM though
that something like V2E wasn't  pursued for KVM.

Regards,

Anthony Liguori

> Eagerly looking forward to your reply!
>
> -Gonglei
>

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