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Date: Fri, 6 Nov 2009 02:29:09 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH 4/4] Add support for -net bridge
Message-ID: <20091106022909.GN21630@shareable.org>
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To: Anthony Liguori <aliguori@us.ibm.com>
Cc: Mark McLoughlin <markmc@redhat.com>, Arnd Bergmann <arndbergmann@googlemail.com>, Dustin Kirkland <kirkland@canonical.com>, Michael Tsirkin <mst@redhat.com>, Juan Quintela <quintela@redhat.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>

Anthony Liguori wrote:
> Avi Kivity wrote:
> >>+int net_init_bridge(QemuOpts *opts, Monitor *mon, const char *name, 
> >>VLANState *vlan);
> >>+
> >>   
> >
> >Don't we need to tear the interface down after shutdown?
> 
> net_init_bridge calls net_tap_fd_init which registers tap_cleanup.  That 
> closes the fd and frees associated memory.
> 
> The helper does not allocate a persistent tap device so closing the file 
> descriptor is sufficient for cleanup.

I think you should at least have the option to call a/the helper when
cleaning up too.

For the same reason that -net downscript= was added.  (From the names
I get the impression that was added later than -net script=, perhaps
due to the same kind of oversight ;-)

A user-supplied helper (I'm thinking of my needs) would create per-tap
iptables/ebtables rules and perhaps routing table entries, in addition
to creating the tap interface itself.  Routing table entries are
automatically deleted when the interface is, but iptables/ebtables
rules are not.  (Although it would be a nice little kernel addition if
they could be flagged to be.)

But now, thinking a bit more clearly... why is the helper separate
from "-net script="?

Overall, I envisage this happening (sorry for making up the name
tap-up-helper, as I seem to have lost your patch mails):

    tap-up-helper=      # Sets up tap interface, adds to bridge if needed.
    script=             # Sets up IP config

       ....QEMU runs....

    downscript=         # Removes IP config
    tap-down-helper=    # If only for symmetry!

We does tap-up-helper need to be separate from script?  Does QEMU need
to do something in between calling the two?

If not, then the existing "-net script=" script could just send the
tap descriptor as well as anything else it does, couldn't it?

-- Jamie