[RFC 2/3] docs/specs: IPMI device emulation: main processor

[Havard Skinnemoen via <qemu-devel@nongnu.org>]

This document is an attempt to briefly document the existing IPMI
emulation support on the main processor. It provides the necessary
background for the BMC-side IPMI emulation proposed by the next patch.

Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
---
 docs/specs/index.rst |   1 +
 docs/specs/ipmi.rst  | 100 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)
 create mode 100644 docs/specs/ipmi.rst

diff --git a/docs/specs/index.rst b/docs/specs/index.rst
index 1b0eb979d5..9fdb94a578 100644
--- a/docs/specs/index.rst
+++ b/docs/specs/index.rst
@@ -14,5 +14,6 @@ Contents:
    ppc-spapr-xive
    ppc-spapr-numa
    acpi_hw_reduced_hotplug
+   ipmi
    tpm
    acpi_hest_ghes
diff --git a/docs/specs/ipmi.rst b/docs/specs/ipmi.rst
new file mode 100644
index 0000000000..e0f9ab20ba
--- /dev/null
+++ b/docs/specs/ipmi.rst
@@ -0,0 +1,100 @@
+=====================
+IPMI device emulation
+=====================
+
+QEMU supports emulating many types of machines. This includes machines that may
+serve as the main processor in an IPMI system, e.g. x86 or POWER server
+processors, as well as machines emulating ARM-based Baseband Management
+Controllers (BMCs), e.g. AST2xxx or NPCM7xxx systems-on-chip.
+
+Main processor emulation
+========================
+
+A server platform may include one of the following system interfaces for
+communicating with a BMC:
+
+* A Keyboard Controller Style (KCS) Interface, accessible via ISA
+  (``isa-ipmi-kcs``) or PCI (``pci-ipmi-kcs``).
+* A Block Transfer (BT) Interface, accessible via ISA (``isa-ipmi-bt``) or PCI
+  (``pci-ipmi-bt``).
+* An SMBus System Interface (SSIF; ``smbus-ipmi``).
+  
+These interfaces can all be emulated by QEMU. To emulate the behavior of the
+BMC, the messaging interface emulators use one of the following backends:
+
+* A BMC simulator running within the QEMU process (``ipmi-bmc-sim``).
+* An external BMC simulator or emulator, connected over a chardev
+  (``ipmi-bmc-extern``). `ipmi_sim
+  <https://github.com/wrouesnel/openipmi/blob/master/lanserv/README.ipmi_sim>`_
+  from OpenIPMI is an example external BMC emulator.
+
+The following diagram shows how these entities relate to each other.
+
+.. blockdiag::
+
+    blockdiag main_processor_ipmi {
+        orientation = portrait
+        default_group_color = "none";
+        class msgif [color = lightblue];
+        class bmc [color = salmon];
+
+        ipmi_sim [color="aquamarine", label="External BMC"]
+        ipmi-bmc-extern <-> ipmi_sim [label="chardev"];
+
+        group {
+            orientation = portrait
+
+            ipmi-interface <-> ipmi-bmc;
+
+            group {
+                orientation = portrait
+
+                ipmi-interface [class = "msgif"];
+                isa-ipmi-kcs [class="msgif", stacked];
+
+                ipmi-interface <- isa-ipmi-kcs [hstyle = generalization];
+            }
+
+
+            group {
+                orientation = portrait
+
+                ipmi-bmc [class = "bmc"];
+                ipmi-bmc-sim [class="bmc"];
+                ipmi-bmc-extern [class="bmc"];
+
+                ipmi-bmc <- ipmi-bmc-sim [hstyle = generalization];
+                ipmi-bmc <- ipmi-bmc-extern [hstyle = generalization];
+            }
+
+        }
+    }
+
+IPMI System Interfaces
+----------------------
+
+The system software running on the main processor may use a *system interface*
+to communicate with the BMC. These are hardware devices attached to an ISA, PCI
+or i2c bus, and in QEMU, they all need to implement ``ipmi-interface``.
+This allows a BMC implementation to interact with the system interface in a
+standard way.
+
+IPMI BMC
+--------
+
+The system interface devices delegate emulation of BMC behavior to a BMC
+device, that is a subclass of ``ipmi-bmc``. This type of device is called
+a BMC because that's what it looks like to the main processor guest software.
+
+The BMC behavior may be simulated within the qemu process (``ipmi-bmc-sim``) or
+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.
+
+Wire protocol
+=============
+
+The wire protocol used between ``ipmi-bmc-extern`` and the external BMC
+emulator is defined by `README.vm
+<https://github.com/wrouesnel/openipmi/blob/master/lanserv/README.vm>`_ from
+the OpenIPMI project.
-- 
2.28.0.709.gb0816b6eb0-goog