On Tue, Mar 11, 2025 at 02:27:14PM +0100, Laurent Vivier wrote: > Add a chapter to explain how to use passt(1) instead of '-net user'. > passt(1) can be connected to QEMU using UNIX socket or vhost-user. > With vhost-user, migration of the VM is allowed and internal state of > passt(1) is transfered from one side to the other > > Bug: https://gitlab.com/qemu-project/qemu/-/issues/2827 > Signed-off-by: Laurent Vivier <lviv...@redhat.com>
Reviewed-by: David Gibson <da...@gibson.dropbear.id.au> > --- > docs/system/devices/net.rst | 100 ++++++++++++++++++++++++++++++++++++ > 1 file changed, 100 insertions(+) > > diff --git a/docs/system/devices/net.rst b/docs/system/devices/net.rst > index 2ab516d4b097..a3efbdcabd1a 100644 > --- a/docs/system/devices/net.rst > +++ b/docs/system/devices/net.rst > @@ -77,6 +77,106 @@ When using the ``'-netdev user,hostfwd=...'`` option, TCP > or UDP > connections can be redirected from the host to the guest. It allows for > example to redirect X11, telnet or SSH connections. > > +Using passt as the user mode network stack > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +passt_ can be used as a simple replacement for SLIRP (``-net user``). > +passt doesn't require any capability or privilege. passt has > +better performance than ``-net user``, full IPv6 support and better security > +as it's a daemon that is not executed in QEMU context. > + > +passt can be connected to QEMU either by using a socket > +(``-netdev stream``) or using the vhost-user interface (``-netdev > vhost-user``). > +See `passt(1)`_ for more details on passt. > + > +.. _passt: https://passt.top/ > +.. _passt(1): https://passt.top/builds/latest/web/passt.1.html > + > +To use socket based passt interface: > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > + > +Start passt as a daemon:: > + > + passt --socket ~/passt.socket > + > +If ``--socket`` is not provided, passt will print the path of the UNIX > domain socket QEMU can connect to (``/tmp/passt_1.socket``, > ``/tmp/passt_2.socket``, > +...). Then you can connect your QEMU instance to passt: > + > +.. parsed-literal:: > + |qemu_system| [...OPTIONS...] -device virtio-net-pci,netdev=netdev0 > -netdev stream,id=netdev0,server=off,addr.type=unix,addr.path=~/passt.socket > + > +Where ``~/passt.socket`` is the UNIX socket created by passt to > +communicate with QEMU. > + > +To use vhost-based interface: > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > + > +Start passt with ``--vhost-user``:: > + > + passt --vhost-user --socket ~/passt.socket > + > +Then to connect QEMU: > + > +.. parsed-literal:: > + |qemu_system| [...OPTIONS...] -m $RAMSIZE -chardev > socket,id=chr0,path=~/passt.socket -netdev vhost-user,id=netdev0,chardev=chr0 > -device virtio-net,netdev=netdev0 -object > memory-backend-memfd,id=memfd0,share=on,size=$RAMSIZE -numa node,memdev=memfd0 > + > +Where ``$RAMSIZE`` is the memory size of your VM ``-m`` and ``-object > memory-backend-memfd,size=`` must match. > + > +Migration of passt: > +^^^^^^^^^^^^^^^^^^^ > + > +When passt is connected to QEMU using the vhost-user interface it can > +be migrated with QEMU and the network connections are not interrupted. > + > +As passt runs with no privileges, it relies on passt-repair to save and > +load the TCP connections state, using the TCP_REPAIR socket option. > +The passt-repair helper needs to have the CAP_NET_ADMIN capability, or run > as root. If passt-repair is not available, TCP connections will not be > preserved. > + > +Example of migration of a guest on the same host > +________________________________________________ > + > +Before being able to run passt-repair, the CAP_NET_ADMIN capability must be > set > +on the file, run as root:: > + > + setcap cap_net_admin+eip ./passt-repair > + > +Start passt for the source side:: > + > + passt --vhost-user --socket ~/passt_src.socket --repair-path > ~/passt-repair_src.socket > + > +Where ``~/passt-repair_src.socket`` is the UNIX socket created by passt to > +communicate with passt-repair. The default value is the ``--socket`` path > +appended with ``.repair``. > + > +Start passt-repair:: > + > + passt-repair ~/passt-repair_src.socket > + > +Start source side QEMU with a monitor to be able to send the migrate command: > + > +.. parsed-literal:: > + |qemu_system| [...OPTIONS...] [...VHOST USER OPTIONS...] -monitor stdio > + > +Start passt for the destination side:: > + > + passt --vhost-user --socket ~/passt_dst.socket --repair-path > ~/passt-repair_dst.socket > + > +Start passt-repair:: > + > + passt-repair ~/passt-repair_dst.socket > + > +Start QEMU with the ``-incoming`` parameter: > + > +.. parsed-literal:: > + |qemu_system| [...OPTIONS...] [...VHOST USER OPTIONS...] -incoming > tcp:localhost:4444 > + > +Then in the source guest monitor the migration can be started:: > + > + (qemu) migrate tcp:localhost:4444 > + > +A separate passt-repair instance must be started for every migration. In the > case of a failed migration, passt-repair also needs to be restarted before > trying > +again. > + > Hubs > ~~~~ > -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson
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