Re: [Qemu-devel] [PATCH v2] net: add the support for -netdev socket, listen

[Stefan Hajnoczi]

On Mon, May 28, 2012 at 03:49:11PM +0800, Zhi Yong Wu wrote:
> On Sun, Feb 26, 2012 at 10:48 PM, Stefan Hajnoczi <address@hidden> wrote:
> > On Sat, Feb 18, 2012 at 9:19 AM,  <address@hidden> wrote:
> >> From: Zhi Yong Wu <address@hidden>
> >>
> >> The -net socket,listen option does not work with the newer -netdev
> >> syntax:
> >> http://lists.gnu.org/archive/html/qemu-devel/2011-11/msg01508.html
> >>
> >> This patch makes it work now.
> >>
> >> Signed-off-by: Zhi Yong Wu <address@hidden>
> >> ---
> >>  net.c        |   26 +++++++++++++++++++++
> >>  net.h        |    2 +
> >>  net/socket.c |   72 
> >> +++++++++++++++++++++++++++++++++++++++++++++-------------
> >>  3 files changed, 84 insertions(+), 16 deletions(-)
> >
> > I wanted to understand the problem better so I tried out -net
> > socket,listen=.  Here is its behavior:
> >
> > 1. A client can connect to QEMU, this creates a new socket
> > VLANClientState on the VLAN.
> > 2. If another client connects to QEMU, another VLANClientState is
> > created.  That means many socket clients can be added to the same
> > "VLAN".
> > 3. When a simple TCP client like netcat connects and then disconnects,
> > the VLANClientState remains forever.  There seems to be no cleanup.
> >
> > This patch does not handle the -net socket,listen= case where multiple
> > clients connect.
> >
> > Also, the -netdev socket,listen= semantics cannot match -net
> > socket-listen= semantics because there is only one peer at any time.
> > Some options:
> >
> > 1. Do not accept new connections while a client is connected.  Once
> > the client disconnects we can accept a new connection.  This maintains
> > the 1-1 peer behavior.
> > 2. Integrate with vlan-hub so that multiple clients can connect even
> > with -netdev.  Connections will create new NetClientStates and
> > auto-attach to the hub.  This mimics -net socket,listen= but requires
> > a hub to be used.
> > 3. Forbid -netdev socket,listen=, only allow -net socket,listen=.
> >
> > I think #1 would be okay, although it no longer allows multiple
> > connections, but I don't have a strong opinion either way.
> Now i prefer to support #2. Do you think of it? Should the usage
> "-netdev socket,listen" also been supported or forbidden? As you said,
> -netdev only has one peer, so their usage will be a bit different.

I'm not sure how useful the multiple connections behavior is.  Since
-netdev socket,listen= has not worked in the past we have the freedom to
decide how it should work (without breaking existing users' setups).

Several folks have pointed out that vde or other external programs are
better for virtual hubs/switches.  I would implement #1 because it adds
useful behavior but doesn't complicate QEMU much.

But if you feel adding #2 would be worthwhile and not a big effort to
code, then go ahead.  My intuition is that #1 will be easier to get
merged and can be extended to support #2 in the future, if necessary.

Stefan