Re: [PATCH v2] docs: Explain how to use passt

[Laurent Vivier <lvivier@redhat.com>]

cc: trivial

On 11/03/2025 14:27, 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 <lvivier@redhat.com>
> ---
>   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
>   ~~~~
>