Re: [RFC] Xen Virtual Framebuffer

[Gerd Knorr <kraxel@suse.de>]

Anthony Liguori wrote:
> Now that's 3.0.0's out, I thought it would be a good time to bring up 
> the topic of framebuffer virtualization.
> 
> I threw together a proof-of-concept over the weekend of a simple virtual 
> framebuffer/keyboard/mouse.  The basic design is have a vmalloc()'d 
> buffer in the guest exposed as /dev/fb0 and mmap()'d in dom0.  There's 
> also a simple message system for keyboard/mouse events.

Browsed through the code, looked at the wiki page now ...

First, I've hacked up something very simliar for user-mode-linux, 
patches are at http://www.suse.de/~kraxel/uml/patches/2.6.15-rc5/, you 
might want to have a look and steel some ideas (unless you already did).
Note that the input system got some sysfs-related changes recently, so 
the patches really work on very latest kernels only.

Some more comments:

  * why do you put the mfn and evtchn into the start page?  IMHO that
    should be handled by xenstore, like it is done for virtual network
    and disk.

  * I would create two different input devices for mouse/keyboard
    (although strictly seen it isn't really needed) to avoid confusing
    userspace apps.

  * I would simply reuse the input event struct from
    include/linux/input.h for dom0 -> domU input event delivery
    instead of creating something new.

  * i18n keyboard is really tricky (that is a huge mess in vmx btw).
    I think the only sane approach is to let the guest (domU) do
    all the i18n handling and try to avoid doing any key mapping
    in the host (dom0).  If you have *two* instances (both host
    and guest) doing keyboard mapping and both not even knowing
    what the other one is doing you have absolutely no chance to
    get it right ...

Keeping track of the in-kernel screen changes (fbcon) is easy, tracking 
userspace is more difficuilt but doable, the uml framebuffer driver does 
that by watching page faults: The no_page handler maps the page and 
marks it "dirty", the update function does zap_pte_range to kill the 
mappings so the no_page handler is called on the next update again.

cheers,
   Gerd