[coreboot] [SPECIFICATION RFC] The firmware and bootloader log specification

[Daniel Kiper]

Hey,

This is next attempt to create firmware and bootloader log specification.
Due to high interest among industry it is an extension to the initial
bootloader log only specification. It takes into the account most of the
comments which I got up until now.

The goal is to pass all logs produced by various boot components to the
running OS. The OS kernel should expose these logs to the user space
and/or process them internally if needed. The content of these logs
should be human readable. However, they should also contain the
information which allows admins to do e.g. boot time analysis.

The log specification should be as much as possible platform agnostic
and self contained. The final version of this spec should be merged into
existing specifications, e.g. UEFI, ACPI, Multiboot2, or be a standalone
spec, e.g. as a part of OASIS Standards. The former seems better but is
not perfect too...

Here is the description (pseudocode) of the structures which will be
used to store the log data.

struct bf_log
  {
    uint32_t   version;
    char       producer[64];
    uint64_t   flags;
    uint64_t   next_bf_log_addr;
    uint32_t   next_msg_off;
    bf_log_msg msgs[];
  }

struct bf_log_msg
  {
    uint32_t size;
    uint64_t ts_nsec;
    uint32_t level;
    uint32_t facility;
    uint32_t msg_off;
    char     strings[];
  }

The members of struct bf_log:
  - version: the firmware and bootloader log format version number, 1 for now,
  - producer: the producer/firmware/bootloader/... type; the length
    allows ASCII UUID storage if somebody needs that functionality,
  - flags: it can be used to store information about log state, e.g.
    it was truncated or not (does it make sense to have an information
    about the number of lost messages?),
  - next_bf_log_addr: address of next bf_log struct; none if zero (I think
    newer spec versions should not change anything in first 5 bf_log members;
    this way older log parsers will be able to traverse/copy all logs regardless
    of version used in one log or another),
  - next_msg_off: the offset, in bytes, from the beginning of the bf_log struct,
    of the next byte after the last log message in the msgs[]; i.e. the offset
    of the next available log message slot; it is equal to the total size of
    the log buffer including the bf_log struct,
  - msgs: the array of log messages,
  - should we add CRC or hash or signatures here?

The members of struct bf_log_msg:
  - size: total size of bf_log_msg struct,
  - ts_nsec: timestamp expressed in nanoseconds starting from 0,
  - level: similar to syslog meaning; can be used to differentiate normal messages
    from debug messages; the exact interpretation depends on the current producer
    type specified in the bf_log.producer,
  - facility: similar to syslog meaning; can be used to differentiate the sources of
    the messages, e.g. message produced by networking module; the exact interpretation
    depends on the current producer type specified in the bf_log.producer,
  - msg_off: the log message offset in strings[],
  - strings[0]: the beginning of log message type, similar to the facility member but
    NUL terminated string instead of integer; this will be used by, e.g., the GRUB2
    for messages printed using grub_dprintf(),
  - strings[msg_off]: the beginning of log message, NUL terminated string.

Note: The producers are free to use/ignore any given set of level, facility and/or
      log type members. Though the usage of these members has to be clearly defined.
      Ignored integer members should be set to 0. Ignored log message type should
      contain an empty NUL terminated string. The log message is mandatory but can
      be an empty NUL terminated string.

There is still not fully solved problem how the logs should be presented to the OS.
On the UEFI platforms we can use config tables to do that. Then probably
bf_log.next_bf_log_addr should not be used. On the ACPI and Device Tree platforms
we can use these mechanisms to present the logs to the OSes. The situation gets more
difficult if neither of these mechanisms are present. However, maybe we should not
bother too much about that because probably these platforms getting less and less
common.

Anyway, I am aware that this is not specification per se. The goal of this email is
to continue the discussion about the idea of the firmware and booloader log and to
find out where the final specification should land. Of course taking into the account
assumptions made above.

You can find previous discussions about related topics at [1], [2] and [3].

Additionally, I am going to present this during GRUB mini-summit session on Tuesday,
17th of November at 15:45 UTC. So, if you want to discuss the log design please join
us. You can find more details here [4].

Daniel

[1] https://lists.gnu.org/archive/html/grub-devel/2019-10/msg00107.html
[2] https://lists.gnu.org/archive/html/grub-devel/2019-11/msg00079.html
[3] https://lists.gnu.org/archive/html/grub-devel/2020-05/msg00223.html
[4] https://twitter.com/3mdeb_com/status/1327278804100931587