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

["Gonglei (Arei)" <arei.gonglei@xxxxxxxxxx>]

I got VMEXIT counts by xentrace When using the VNC protocol grab 
the RDP to connect Windows XP guest. the time-interval of running 
xentrace is five seconds.The number of VMEXIT (EXIT_REASON_EPT_VIOLATION)
in unstream qemu far greater than qemu-dm.
 
the command likes: 
   xentrace -D -T 5 -c 0x03 -e 0x0008f000 data/xentrace_$dt.log
   
The results of qemu upstream:

linux-sAGhxH:/home/sdb/gonglei/xentrace # ./xentrace.sh 
change evtmask to 0x8f000
change cpumask to 0x3
Start analyze xentrace-format_2013-0726-1444.log ...
  VMExitCode                            VMExitReason      VMExitCnt         
VMExitTicks
0x0000000000               EXIT_REASON_EXCEPTION_NMI            447             
 329716
0x0000000001          EXIT_REASON_EXTERNAL_INTERRUPT           1126             
3039311
0x0000000007           EXIT_REASON_PENDING_VIRT_INTR           1865             
1608939
0x000000001c                   EXIT_REASON_CR_ACCESS            350             
 913047
0x000000001d                   EXIT_REASON_DR_ACCESS              4             
   8266
0x000000001e              EXIT_REASON_IO_INSTRUCTION           7669           
258978672
0x000000002b         EXIT_REASON_TPR_BELOW_THRESHOLD           1203             
1535626
0x000000002c                 EXIT_REASON_APIC_ACCESS          12971           
106012627
0x0000000030               EXIT_REASON_EPT_VIOLATION         657459          
5808859252
Total                                                        683094          
6181285456

The results of traditional qemu-dm:  
 
linux-sAGhxH:/home/sdb/gonglei/xentrace # ./xentrace.sh 
change evtmask to 0x8f000
change cpumask to 0xc
Start analyze xentrace-format_2013-0726-1451.log ...
  VMExitCode                            VMExitReason      VMExitCnt         
VMExitTicks
0x0000000000               EXIT_REASON_EXCEPTION_NMI           8693             
6344766
0x0000000001          EXIT_REASON_EXTERNAL_INTERRUPT           3587            
10601657
0x0000000007           EXIT_REASON_PENDING_VIRT_INTR           8086             
6555873
0x000000000a                       EXIT_REASON_CPUID             34             
  56060
0x000000001c                   EXIT_REASON_CR_ACCESS           1424             
3757518
0x000000001d                   EXIT_REASON_DR_ACCESS             13             
  27297
0x000000001e              EXIT_REASON_IO_INSTRUCTION          21250           
492487340
0x000000002b         EXIT_REASON_TPR_BELOW_THRESHOLD           6458             
8184132
0x000000002c                 EXIT_REASON_APIC_ACCESS          61749           
506233523
0x0000000030               EXIT_REASON_EPT_VIOLATION          20329           
261274392
0x0000000036                      EXIT_REASON_WBINVD              2             
   2027
Total                                                        131625          
1295524585

-Gonglei

> -----Original Message-----
> From: Gonglei (Arei)
> Sent: Friday, July 26, 2013 2:41 PM
> To: 'Anthony Liguori'
> 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
> 
> 
> 
> > -----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 ?
> 
> 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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