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docs: Explain how to use passt

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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 <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
This commit is contained in:
Laurent Vivier 2025-03-11 14:27:14 +01:00 • committed by Michael Tokarev
parent 14fb6dbbc5
commit 94a9471ceb
1 changed files with 100 additions and 0 deletions
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100
docs/system/devices/net.rst
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@ -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
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