[Qemu-devel] cirrus logic pathological case slow screen update

[Stefan Hajnoczi <stefanha@gmail.com>]

Hi,
I've just seen a system running Windows 2008 64-bit under KVM doing a
Windows Update.  It was applying 1000s of updates in a text mode that
triggers a pathological case with the cirrus logic emulation (I
connected over VNC).

The symptom is that the message being displayed "Applying update 1 out
of 11666" is redrawn at an extremely slow speed - maybe one second to
render the line of text.  You can see portions of letters being
blitted to the screen.  The virtual machine appears blocked by this
screen redraw so the updates apply *very* slowly.

When -vga is changed from cirrus to std the issue goes away and the
update proceeds at normal speed.

I was given temporary VNC access to the problem VM so I don't know all
the configuration details, but I did see the issue and that it was
fixed by -vga cirrus -> -vga std.  Any ideas?

If it happens again perhaps the KVM stats will tell us that it is
emulating framebuffer memory accesses or something very expensive.  I
didn't get a chance to try but would this be possible with cirrus and
not std?

Stefan