Red Hat Bugzilla – Bug 493630
Provide support for configuring a working mouse pointer for the Xen VNC console.
Last modified: 2009-07-31 10:45:00 EDT
Created attachment 337803 [details]
Patch to recognise and configure a xen virtual keyboard/pointer when present
There is a long-standing problem of mouse pointer tracking with the Xen VNC console. This typically shows up as the local VNC cursor (for the local application) and the remote cursor being out of sync -- in some cases the local and remote cursors are identical and this makes driving the Xen VNC console very difficult.
The problem can be partially solved by using a custom VNC client: the gtk vncviewer widget does just that and by completely hiding the local cursor masks the problem quite effectively. However, there are still problems because the remote cursor does not move at the speed the user is expecting (mostly it's too slow, but when it's too fast it's unusable) and, without some careful trickery, the remote cursor can be seen to bump into a "wall" when the local cursor hits the edge of the screen. Of course, this solution does not work at all unless you're using that particular VNC client -- which you can't on Windows.
Fortunately, there is a way out of this. The Xen virtual pointer device allows you to use absolute addressing (like a tablet) and if you do this then you don't have mismatched cursors, speed or invisible wall problems. It works so well that within a couple of seconds you're left wondering how you ever managed before.
There are a couple of problems to get around. First, because of the combination pointer/keyboard virtual device (see bug 493618) the configuration is, to say the least, complex and difficult. Second, again because of the combo pointer/keyboard device you need to add xkb support to the evdev driver (bug 493623). There are also problems with correctly activating the keyboard in rhpl (bug on its way) and the evdev xkb rules (bug 493627) but these are relatively minor compared to getting the configuration right.
The configuration complexities are mostly taken care of by rhpxl. With these changes in place you need only run "system-config-display --reconfig" and you'll get a working X server configuration.
If the fixes the rhpxl, rhpl, xkeyboard-config and xorg-x11-drv-evdev are retrofitted into a distribution DVD then you also get a properly tracking mouse during installation. Several of the people I've spoken to are driven potty by the poor mouse tracking during installation and are glad to be able to forget the graphical environment once they've done -- yes I know that you and I would get around this with using "vnc" on the installation boot command, but not enough people know that. With this fix, it just, finally, and blissfully works.
Version-Release number of selected component (if applicable): 0.41.1
How reproducible: Always
Steps to Reproduce:
1. Install a Xen PV guest. Use a "standard" vnc viewer rather than the gtk vncviewer that masks some, but not all of the problems. You want to experience the full horror of the problem.
2. Attempt to move the cursor in a useful fashion
Actual results: Mouse cursor varies between merely annoying and downright impossible
Expected results: The local and remote cursors should track perfectly
Additional info: It's amazing how much easier a Xen PV graphical installation is once you've fixed this, not even the gtk vnc viewer comes close to getting a properly tracking mouse.
Note that you can't use kudzu to discover the xen virtual keyboard/pointer; probably because it's a virtual device, not a real one.
Related bugs: bug 493618, bug 493623, bug 493627, bug 493630, bug 493634 and bug 493642
The bug as reported is about the difficulties to manually configure X
in a certain way, to get proper pointer tracking. It is certainly
valid. However, the real problem users care about is that pointer
tracking works very poorly (mouse hits invisible wall).
Upstram, we solved the problem by fixing numerous issues in various
places to make it work out of the box, from Fedora 11 on.
The patches proposed by the reporter solve the problem by making
manual configuration work. They are unrelated to the upstream
Like the upstream solution, they're fairly invasive: they touch the
kernel and several user space packages. This is a significant risk.
Moreover, splitting the device is an ABI change. The question is
whether there are benefits to justify that.
We already put much less invasive changes into 5.4 to improve pointer
tracking (bug 492866). They fix the "mouse hits invisible wall"
problem. They still require a pointer grab, which virt-viewer makes
So, the benefit of the proposed patches over what we've got already is
to enable manual configuration so that the mouse grab can be avoided.
While that's certainly nice, it doesn't justify the risk, in my
If you disagree, please reopen the bug.
Some technical background:
There are 3 pieces in the stack: VNC client, QEMU VNC server and the
guest's X. The old, misbehaving setup has VNC client sending absolute
coordinates, which the VNC server sends through xenkbd to X. X in
RHEL-5 doesn't have any auto-configuration of input devices, so it
just opens /dev/input/mice, and thus the guest kernel converts from
absolute to relative. Because the VNC client has no knowledge of this
conversion, you get into situation where you hit an invisible wall in
the client when it thinks it has got to the virtual desktop boundary.
Now, if you had absolute coordinates being passed and used all the way
to X then it would trivially work, but this requires X configuration.
Recent X's do that automatically. RHEL-5's X simply isn't capable of
that. The patches proposed by the reporter add manual configuration
steps across a wide range of RPMs.
The fix we did for RHEL-5.4 is to make xenkbd *not* use absolute
coordinates, so the VNC server now sends relative coordinates. This
removes the broken absolute -> relative conversion in the guest
kernel. On its own this isn't sufficient, because you then get broken
absolute to relative conversion in the VNC server instead. So we also
added the VNC relative mouse extension to QEMU and GTK-VNC. Now the
VNC client is in charge of doing the absolute -> relative conversion.
This is good, as this is the only point in the stack capable of doing
the conversion correctly, to avoid hitting an invisible wall. The
only restriction is that you must grab the host mouse pointer and hide
it, so you only see the guest drawn pointer.
By the way, RHEL-6 pointer tracking will be fine out of the box using absolute coordinates, just like upstream.