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

[Gerd Hoffmann <kraxel@redhat.com>]

On 07/26/13 12:02, Andreas Färber wrote:
> Am 26.07.2013 08:41, schrieb Gonglei (Arei):
>>> -----Original Message-----
>>> From: Anthony Liguori [mailto:anthony@codemonkey.ws]
>>> Sent: Friday, July 26, 2013 11:21 AM
>>> To: Gonglei (Arei)
>>> Cc: qemu-devel@nongnu.org; xen-devel@lists.xen.org; Hanweidong;
>>> Luonengjun; Huangweidong (Hardware)
>>> Subject: Re: [Qemu-devel] Cirrus VGA slow screen update, show blank screen
>>> last 13s or so for windows XP guest
>>>
>>>  On Thu, Jul 25, 2013 at 10:08 PM, Gonglei (Arei)
>>> <arei.gonglei@huawei.com> wrote:
>>>> 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.
>>
>> Perhaps I did not express clearly what I mean at #3.
>> I don't understand why the qemu-dm(qemu-0.10.2) works well 
>> but the upstream qemu(begin with qemu-0.14) is not for the 
>> same windows XP guest image in cirrus vga emulation. Does 
>> the cirrus vga emulation have some differences between 
>> qemu-dm and unstream qemu ?
> 
> I don't know about qemu-dm, but QEMU switched from a home-grown pixel
> handling to pixman library sometime around 1.4. CC'ing Gerd.

0.14 != 1.4 ;)

Also pixman isn't used everywhere, there is still home-grown pixel
handling code in qemu.  I often replace it when I touch it for some
reason, but we are far away from a full-pixman qemu.  The cirrus blit
ops havn't been touched btw, it's still the old code.

Maybe the xen guys did some optimizations in qemu-dm which where not
merged upstream.  Try asking @ xen-devel.

Beside that the standard vga usually is the better choice anyway as it
supports more video memory and higher resolutions than cirrus.

cheers,
  Gerd