docs/designs/bios-bmc-smm-error-logging.md

1 # BIOS->BMC SMM Error Logging Queue Daemon
16 (SMM) to provide error logs to the BMC, requiring messages to be sent as quickly
17 as possible without a handshake / ack back from the BMC due to the time
19 implement a circular buffer over a shared BIOS->BMC buffer that the BIOS can
24 There are various ways of communicating between the BMC and the BIOS, but there
28 Different BMC vendors support different methods such as Shared Memory (SHM, via
38 1. The BMC shall initialize the shared buffer in a way that the BIOS can
41    the BMC before any writes to the shared buffer (**no synchronization**)
42 3. The BIOS shall be the main writer to the shared buffer, with the BMC mainly
44 4. The BMC shall read new payloads from the shared buffer for further processing
47 The shared buffer will be as big as the protocol allows for a given BMC platform
68 …BMC Interface Version               | 4 bytes                          | 0x0         | BMC at init…
69 …4 bytes                          | 0x4         | BIOS at init | Allows the BMC to determine if it …
70 …BMC at init  | Magic number to set the state of the queue as described below. Written by BMC once …
71 …            | 0x18        | BMC at init  | Indicates the size of the region allocated for the circ…
72 …       | 0x1b        | BMC at init  | Indicates the size of the region reserved for Uncorrectable …
73 …BMC flags                           | 4 bytes                          | 0x1d        | BMC        …
74 | BMC read pointer                    | 3 bytes                          | 0x21        | BMC       …
76 …BMC know that it missed an error log</li><li>Toggled when BIOS sees overflow and not already overf…
77 …      | Indicates where the next log will be written by BIOS. Used to tell BMC when it should read…
85 buffer, then initializing the header's `BMC at init` fields before writing the
90 If there are any further initialization between the BIOS and the BMC required,
91 the BMC needs to set the `BMC_READY` bit in the BMC flags once the
93 shall set the `Incomplete Initialization` flag to notify the BMC to reinitialize
107 overflow of the normal error log queue is imminent and the BMC has acked that
109 `BMC flag`.
122 - LPC / eSPI was also considered but our BMC's SHM buffer was limited to 4KB
129     with expertise both from the BMC and the BIOS side (which we currently lack)
134 Reading from the buffer and processing it may hinder performance of the BMC,
141 - bios-bmc-smm-error-logger