@@ -91,6 +91,76 @@ further delegated to an external emulator, or a real BMC. The
``ipmi-bmc-extern`` device has a required ``chardev`` property which specifies
the communications channel to the external BMC.
+Baseband Management Controller (BMC) emulation
+==============================================
+
+This section is about emulation of IPMI-related devices in a System-on-Chip
+(SoC) used as a Baseband Management Controller. This is not to be confused with
+emulating the BMC device as seen by the main processor.
+
+SoCs that are designed to be used as a BMC often have dedicated hardware that
+allows them to be connected to one or more of the IPMI System Interfaces. The
+BMC-side hardware interface is not standardized, so each type of SoC may need
+its own device implementation in QEMU, for example:
+
+* ``aspeed-ibt`` for emulating the Aspeed iBT peripheral.
+* ``npcm7xx-kcs`` for emulating the Nuvoton NPCM7xx Host-to-BMC Keyboard
+ Controller Style (KCS) channels.
+
+.. blockdiag::
+
+ blockdiag bmc_ipmi {
+ orientation = portrait
+ default_group_color = "none";
+ class interface [color = lightblue];
+ class host [color = salmon];
+
+ host [color="aquamarine", label="External Host"]
+
+ group {
+ orientation = portrait
+
+ group {
+ orientation = portrait
+
+ bmc-interface [class = "interface"]
+ npcm7xx-ipmi-kcs [class = "interface", stacked]
+
+ bmc-interface <- npcm7xx-ipmi-kcs [hstyle = generalization];
+ }
+
+ group {
+ orientation = portrait
+
+ bmc-host [class = "host"];
+ bmc-host-sim [class = "host"];
+ bmc-host-extern [class = "host"];
+
+ bmc-host <- bmc-host-sim [hstyle = generalization];
+ bmc-host <- bmc-host-extern [hstyle = generalization];
+ }
+
+ bmc-interface <-> bmc-host
+ }
+
+ bmc-host-extern <-> host [label="chardev"];
+ }
+
+IPMI Host
+---------
+
+Mirroring the main processor emulation, the interface devices delegate
+emulation of host behavior to a Host device that is a subclass of
+``ipmi-core``. This type of device is called a Host because that's what it
+looks like to the BMC guest software.
+
+The host behavior may be further delegated to an external emulator (e.g.
+another QEMU VM) through the ``ipmi-host-extern`` host implementation. This
+device has a required ``chardev`` property which specifies the communications
+channel to the external host and a required ``interface`` property which
+specifies the underlying IPMI interface. The wire format is the same as for
+``ipmi-bmc-extern``.
+
Wire protocol
=============