Xen devel - Aug 2006 - [patch/rfc] xenbus and driver issues with kexec

Hi,

There are a number of problems in xenbus and the paravirtualized drivers
when it comes to kexec''ing a domU kernel.  It''s not specific
to kexec,
but to handing over devices to someone else within the same domU.  The
same issues will come up when someone tries to combine mini-os and grub
to build some kind of in-domain bootloader.

Fundamental issue is that there is no way to cleanly shutdown the
frontend/backend drivers without destroying the domain and the backend
devices.  The patch addresses some but not all of the problems arising
from that.  It also has some debug stuff left in and needs some more
polishing before final submission.

Here are the changes:

  (1) ballon: print unconditionally driver_pages in /proc/xen/balloon.
  (2) xenbus: unregister watches in xs_suspend()
  (3) blk/netback: allow Closing -> Initialising state transition.
  (4) blk/netfront: shutdown (but *not* destroy) devices
      when devices_shutdown is called, by jumping from Closing to
      Initialising state.
  (5) Some cleanups along the way.

Not solved yet:

The netfront driver leaks pages in case it is shutdown while the
interface is active.  Problem is that it can''t get back the rx buffers,
the netback driver sits on them and doesn''t release them until it has
some packet data to fill in.  The patch adds a netif_release_rx_bufs()
function, but it doesn''t work reliable, usually it gets back only a few
pages.

To reproduce this you don''t need to play with kexec.  Just apply the
balloon driver patch.  Boot a domU with the new kernel.  Configure the
eth0 interface, "xm network-detach ...", "xm
network-attach", repeat.
Watch driver_pages grow in /proc/xen/balloon.

cheers,

  Gerd

PS: documentation and complete domU kexec patches are available from
    http://www.suse.de/~kraxel/xen/kexec.html

-- 
Gerd Hoffmann <kraxel@suse.de>
http://www.suse.de/~kraxel/julika-dora.jpeg


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