[Qemu-devel] [patch] robust user-mode networking

[John Coiner <jcoiner@stanfordalumni.org>]

OK, maybe "robust" should read "slightly less flimsy" :)

It'd be nice if these use cases worked, under user-mode networking:

  1. The user disables and re-enables a network interface in windows. 
Instead of hanging, it should come back up. (Does anyone else have this 
issue?)
  2. The host's DNS server changes. QEMU should reread the host's DNS, 
instead of requiring the user to quit QEMU and run it again. This is an 
issue on a laptop.

The patch below fixes both of these. Handling use case (2) is just a few 
lines -- It rereads host DNS info on each guest DHCP request.

The fix for use case (1) is thousands of lines, this may be controversial...

I transformed the slirp code into a reentrant version of itself. So, 
when bringing up a network card, we can dynamically allocate a slirp 
object for it. If that network card is reset, we can deallocate its 
slirp stack and allocate a fresh one.

   http://people.brandeis.edu/~jcoiner/qemu_idedma/usermode_net.patch

This required lots of typing and no thinking. When it was done, it worked.

The alternative would have been to debug the slirp stack, and understand 
how it works and why Windows leaves it in a frozen state, when disabling 
and reenabling the NIC. That might still be a good idea, considering 
Linux did not seem to have the same problem. Network experts: would you 
expect the slirp stack to be able to restart, mid-stream, without being 
explicitly told to restart? Is there a slirp design document that would 
shed light on this?

I tested the patch with Windows and Linux clients, with the '-smb' and 
'-tftp' options, on a linux host. The build is also tested on a Windows 
host.

This patch includes two other small changes in "vl.c" which I posted to 
the list a few days ago:
  * the fix to make "-smb" use the user's account rather than "nobody"
  * the fix for when the CPU tick counter leaps backward following 
suspend-to-disk.
Should I split those out?

Please let me know if this patch is helpful, or if it introduces bugs, 
or if there would have been an easier way :)

Thanks.

-- John