docs: Add CanoKey documentation

Signed-off-by: Hongren (Zenithal) Zheng <i@zenithal.me>
Message-Id: <YoY6ilQimrK+l5NN@Sun>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/system/device-emulation.rst

@@ -92,3 +92,4 @@ Emulated Devices
    devices/vhost-user.rst
    devices/virtio-pmem.rst
    devices/vhost-user-rng.rst
+   devices/canokey.rst

docs/system/devices/canokey.rst

+.. _canokey:
+
+CanoKey QEMU
+------------
+
+CanoKey [1]_ is an open-source secure key with supports of
+
+* U2F / FIDO2 with Ed25519 and HMAC-secret
+* OpenPGP Card V3.4 with RSA4096, Ed25519 and more [2]_
+* PIV (NIST SP 800-73-4)
+* HOTP / TOTP
+* NDEF
+
+All these platform-independent features are in canokey-core [3]_.
+
+For different platforms, CanoKey has different implementations,
+including both hardware implementions and virtual cards:
+
+* CanoKey STM32 [4]_
+* CanoKey Pigeon [5]_
+* (virt-card) CanoKey USB/IP
+* (virt-card) CanoKey FunctionFS
+
+In QEMU, yet another CanoKey virt-card is implemented.
+CanoKey QEMU exposes itself as a USB device to the guest OS.
+
+With the same software configuration as a hardware key,
+the guest OS can use all the functionalities of a secure key as if
+there was actually an hardware key plugged in.
+
+CanoKey QEMU provides much convenience for debuging:
+
+* libcanokey-qemu supports debuging output thus developers can
+  inspect what happens inside a secure key
+* CanoKey QEMU supports trace event thus event
+* QEMU USB stack supports pcap thus USB packet between the guest
+  and key can be captured and analysed
+
+Then for developers:
+
+* For developers on software with secure key support (e.g. FIDO2, OpenPGP),
+  they can see what happens inside the secure key
+* For secure key developers, USB packets between guest OS and CanoKey
+  can be easily captured and analysed
+
+Also since this is a virtual card, it can be easily used in CI for testing
+on code coping with secure key.
+
+Building
+========
+
+libcanokey-qemu is required to use CanoKey QEMU.
+
+.. code-block:: shell
+
+    git clone https://github.com/canokeys/canokey-qemu
+    mkdir canokey-qemu/build
+    pushd canokey-qemu/build
+
+If you want to install libcanokey-qemu in a different place,
+add ``-DCMAKE_INSTALL_PREFIX=/path/to/your/place`` to cmake below.
+
+.. code-block:: shell
+
+    cmake ..
+    make
+    make install # may need sudo
+    popd
+
+Then configuring and building:
+
+.. code-block:: shell
+
+    # depending on your env, lib/pkgconfig can be lib64/pkgconfig
+    export PKG_CONFIG_PATH=/path/to/your/place/lib/pkgconfig:$PKG_CONFIG_PATH
+    ./configure --enable-canokey && make
+
+Using CanoKey QEMU
+==================
+
+CanoKey QEMU stores all its data on a file of the host specified by the argument
+when invoking qemu.
+
+.. parsed-literal::
+
+    |qemu_system| -usb -device canokey,file=$HOME/.canokey-file
+
+Note: you should keep this file carefully as it may contain your private key!
+
+The first time when the file is used, it is created and initialized by CanoKey,
+afterwards CanoKey QEMU would just read this file.
+
+After the guest OS boots, you can check that there is a USB device.
+
+For example, If the guest OS is an Linux machine. You may invoke lsusb
+and find CanoKey QEMU there:
+
+.. code-block:: shell
+
+    $ lsusb
+    Bus 001 Device 002: ID 20a0:42d4 Clay Logic CanoKey QEMU
+
+You may setup the key as guided in [6]_. The console for the key is at [7]_.
+
+Debuging
+========
+
+CanoKey QEMU consists of two parts, ``libcanokey-qemu.so`` and ``canokey.c``,
+the latter of which resides in QEMU. The former provides core functionality
+of a secure key while the latter provides platform-dependent functions:
+USB packet handling.
+
+If you want to trace what happens inside the secure key, when compiling
+libcanokey-qemu, you should add ``-DQEMU_DEBUG_OUTPUT=ON`` in cmake command
+line:
+
+.. code-block:: shell
+
+    cmake .. -DQEMU_DEBUG_OUTPUT=ON
+
+If you want to trace events happened in canokey.c, use
+
+.. parsed-literal::
+
+    |qemu_system| --trace "canokey_*" \\
+        -usb -device canokey,file=$HOME/.canokey-file
+
+If you want to capture USB packets between the guest and the host, you can:
+
+.. parsed-literal::
+
+    |qemu_system| -usb -device canokey,file=$HOME/.canokey-file,pcap=key.pcap
+
+Limitations
+===========
+
+Currently libcanokey-qemu.so has dozens of global variables as it was originally
+designed for embedded systems. Thus one qemu instance can not have
+multiple CanoKey QEMU running, namely you can not
+
+.. parsed-literal::
+
+    |qemu_system| -usb -device canokey,file=$HOME/.canokey-file \\
+         -device canokey,file=$HOME/.canokey-file2
+
+Also, there is no lock on canokey-file, thus two CanoKey QEMU instance
+can not read one canokey-file at the same time.
+
+Another limitation is that this device is not compatible with ``qemu-xhci``,
+in that this device would hang when there are FIDO2 packets (traffic on
+interrupt endpoints). If you do not use FIDO2 then it works as intended,
+but for full functionality you should use old uhci/ehci bus and attach canokey
+to it, for example
+
+.. parsed-literal::
+
+   |qemu_system| -device piix3-usb-uhci,id=uhci -device canokey,bus=uhci.0
+
+References
+==========
+
+.. [1] `<https://canokeys.org>`_
+.. [2] `<https://docs.canokeys.org/userguide/openpgp/#supported-algorithm>`_
+.. [3] `<https://github.com/canokeys/canokey-core>`_
+.. [4] `<https://github.com/canokeys/canokey-stm32>`_
+.. [5] `<https://github.com/canokeys/canokey-pigeon>`_
+.. [6] `<https://docs.canokeys.org/>`_
+.. [7] `<https://console.canokeys.org/>`_