[PATCH 3/8] docs/specs: IPMI device emulation: BMC

[Hao Wu <wuhaotsh@google.com>]

From: Havard Skinnemoen <hskinnemoen@google.com>

The IPMI document is expanded with a proposal to emulate BMC-side IPMI
devices. This allows a QEMU instance running server software to interact
with a different QEMU instance running BMC firmware, which should
closely model how a real server system works.

Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Signed-off-by: Hao Wu <wuhaotsh@google.com>
---
 docs/specs/ipmi.rst | 70 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/docs/specs/ipmi.rst b/docs/specs/ipmi.rst
index adb098b53d..b5fe362043 100644
--- a/docs/specs/ipmi.rst
+++ b/docs/specs/ipmi.rst
@@ -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
 =============
 
-- 
2.33.0.309.g3052b89438-goog