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

[Fabio Fantoni <fabio.fantoni@xxxxxxx>]

Il 26/07/2013 08:41, Gonglei (Arei) ha scritto:
> > -----Original Message-----
> > From: Anthony Liguori [mailto:anthony@xxxxxxxxxxxxx]
> > Sent: Friday, July 26, 2013 11:21 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
> >
> >   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.
> 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 ?
Probably can be useful try to see here:
https://github.com/xenserver/qemu-xen-4.2.pg/tree/master/master
Probably there are one or more patches not upstreamed that can solve one 
or more problems.
> Thank you so much!
>
> -Gonglei
> > 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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