Mux is a character backend (host side) device, which multiplexes multiple frontends with one backend device. The following is a few lines from the QEMU manpage [1]:
A multiplexer is a "1:N" device, and here the "1" end is your specified chardev backend, and the "N" end is the various parts of QEMU that can talk to a chardev. But sadly multiple backends are not supported. This work implements multiplexing capability of several backend devices, which opens up an opportunity to use a single frontend device on the guest, which can be manipulated from several backend devices. The motivation is the EVE project [2], where it would be very convenient to have a virtio console frontend device on the guest that can be controlled from multiple backend devices. The following is an example of the QEMU command line: -chardev mux-be,id=mux0 \ -chardev socket,path=/tmp/sock,server=on,wait=off,id=sock0,mux-be-id=mux0 \ -chardev vc,id=vc0,mux-be-id=mux0 \ -device virtconsole,chardev=mux0 \ -vnc 0.0.0.0:0 Which creates 2 backend devices: text virtual console (`vc0`) and a socket (`sock0`) connected to the single virtio hvc console with the backend multiplexer (`mux0`) help. `vc0` renders text to an image, which can be shared over the VNC protocol. `sock0` is a socket backend which provides biderectional communication to the virtio hvc console. New type of multiplexer `mux-be` actually is an alias for the same `MuxChardev` struct, which uses same functions as for the original `mux` type, but supports multiplexing N backends with 1 frontend. Once QEMU starts VNC client and any TTY emulator can be used to control a single hvc console, for example these two different consoles should have similar input and output due the buffer multiplexing: # VNC client vncviewer :0 # TTY emulator socat unix-connect:/tmp/sock pty,link=/tmp/pty tio /tmp/pty Difference to the previous version: * Separate type for the backend multiplexer `mux-be` * Handle EAGAIN on write to the backend device * Support of watch of previously failed backend device * Proper json support of the `mux-be-id` option * Unit test for the `mux-be` multiplexer [1] https://www.qemu.org/docs/master/system/qemu-manpage.html#hxtool-6 [2] https://github.com/lf-edge/eve Roman Penyaev (5): chardev/char: introduce `mux-be-id=ID` option and _MUX_BE type chardev/char: rename `mux_cnt` to `fe_cnt` for the `MuxChardev` chardev/char-mux: implement backend chardev multiplexing tests/unit/test-char: add unit test for the `mux-be` multiplexer qemu-options.hx: describe multiplexing of several backend devices chardev/char-fe.c | 14 ++- chardev/char-mux.c | 212 +++++++++++++++++++++++++++++++----- chardev/char.c | 57 ++++++++-- chardev/chardev-internal.h | 33 +++++- include/chardev/char.h | 1 + qapi/char.json | 9 +- qemu-options.hx | 46 +++++++- tests/unit/test-char.c | 217 ++++++++++++++++++++++++++++++++++++- 8 files changed, 538 insertions(+), 51 deletions(-) Signed-off-by: Roman Penyaev <r.peni...@gmail.com> Cc: "Marc-André Lureau" <marcandre.lur...@redhat.com> Cc: qemu-devel@nongnu.org -- 2.43.0
