Documentation/powerpc/firmware-assisted-dump.rst

======================
Firmware-Assisted Dump
======================

July 2011

The goal of firmware-assisted dump is to enable the dump of
a crashed system, and to do so from a fully-reset system, and
to minimize the total elapsed time until the system is back
in production use.

- Firmware assisted dump (fadump) infrastructure is intended to replace
  the existing phyp assisted dump.
- Fadump uses the same firmware interfaces and memory reservation model
  as phyp assisted dump.
- Unlike phyp dump, fadump exports the memory dump through /proc/vmcore
  in the ELF format in the same way as kdump. This helps us reuse the
  kdump infrastructure for dump capture and filtering.
- Unlike phyp dump, userspace tool does not need to refer any sysfs
  interface while reading /proc/vmcore.
- Unlike phyp dump, fadump allows user to release all the memory reserved
  for dump, with a single operation of echo 1 > /sys/kernel/fadump_release_mem.
- Once enabled through kernel boot parameter, fadump can be
  started/stopped through /sys/kernel/fadump_registered interface (see
  sysfs files section below) and can be easily integrated with kdump
  service start/stop init scripts.

Comparing with kdump or other strategies, firmware-assisted
dump offers several strong, practical advantages:

-  Unlike kdump, the system has been reset, and loaded
   with a fresh copy of the kernel.  In particular,
   PCI and I/O devices have been reinitialized and are
   in a clean, consistent state.
-  Once the dump is copied out, the memory that held the dump
   is immediately available to the running kernel. And therefore,
   unlike kdump, fadump doesn't need a 2nd reboot to get back
   the system to the production configuration.

The above can only be accomplished by coordination with,
and assistance from the Power firmware. The procedure is
as follows:

-  The first kernel registers the sections of memory with the
   Power firmware for dump preservation during OS initialization.
   These registered sections of memory are reserved by the first
   kernel during early boot.

-  When a system crashes, the Power firmware will save
   the low memory (boot memory of size larger of 5% of system RAM
   or 256MB) of RAM to the previous registered region. It will
   also save system registers, and hardware PTE's.

   NOTE:
         The term 'boot memory' means size of the low memory chunk
         that is required for a kernel to boot successfully when
         booted with restricted memory. By default, the boot memory
         size will be the larger of 5% of system RAM or 256MB.
         Alternatively, user can also specify boot memory size
         through boot parameter 'crashkernel=' which will override
         the default calculated size. Use this option if default
         boot memory size is not sufficient for second kernel to
         boot successfully. For syntax of crashkernel= parameter,
         refer to Documentation/admin-guide/kdump/kdump.rst. If any offset is
         provided in crashkernel= parameter, it will be ignored
         as fadump uses a predefined offset to reserve memory
         for boot memory dump preservation in case of a crash.

-  After the low memory (boot memory) area has been saved, the
   firmware will reset PCI and other hardware state.  It will
   *not* clear the RAM. It will then launch the bootloader, as
   normal.

-  The freshly booted kernel will notice that there is a new
   node (ibm,dump-kernel) in the device tree, indicating that
   there is crash data available from a previous boot. During
   the early boot OS will reserve rest of the memory above
   boot memory size effectively booting with restricted memory
   size. This will make sure that the second kernel will not
   touch any of the dump memory area.

-  User-space tools will read /proc/vmcore to obtain the contents
   of memory, which holds the previous crashed kernel dump in ELF
   format. The userspace tools may copy this info to disk, or
   network, nas, san, iscsi, etc. as desired.

-  Once the userspace tool is done saving dump, it will echo
   '1' to /sys/kernel/fadump_release_mem to release the reserved
   memory back to general use, except the memory required for
   next firmware-assisted dump registration.

   e.g.::

     # echo 1 > /sys/kernel/fadump_release_mem

Please note that the firmware-assisted dump feature
is only available on Power6 and above systems with recent
firmware versions.

Implementation details:
-----------------------

During boot, a check is made to see if firmware supports
this feature on that particular machine. If it does, then
we check to see if an active dump is waiting for us. If yes
then everything but boot memory size of RAM is reserved during
early boot (See Fig. 2). This area is released once we finish
collecting the dump from user land scripts (e.g. kdump scripts)
that are run. If there is dump data, then the
/sys/kernel/fadump_release_mem file is created, and the reserved
memory is held.

If there is no waiting dump data, then only the memory required
to hold CPU state, HPTE region, boot memory dump and elfcore
header, is usually reserved at an offset greater than boot memory
size (see Fig. 1). This area is *not* released: this region will
be kept permanently reserved, so that it can act as a receptacle
for a copy of the boot memory content in addition to CPU state
and HPTE region, in the case a crash does occur. Since this reserved
memory area is used only after the system crash, there is no point in
blocking this significant chunk of memory from production kernel.
Hence, the implementation uses the Linux kernel's Contiguous Memory
Allocator (CMA) for memory reservation if CMA is configured for kernel.
With CMA reservation this memory will be available for applications to
use it, while kernel is prevented from using it. With this fadump will
still be able to capture all of the kernel memory and most of the user
space memory except the user pages that were present in CMA region::

  o Memory Reservation during first kernel

  Low memory                                         Top of memory
  0      boot memory size                                       |
  |           |                |<--Reserved dump area -->|      |
  V           V                |   Permanent Reservation |      V
  +-----------+----------/ /---+---+----+-----------+----+------+
  |           |                |CPU|HPTE|  DUMP     |ELF |      |
  +-----------+----------/ /---+---+----+-----------+----+------+
        |                                           ^
        |                                           |
        \                                           /
         -------------------------------------------
          Boot memory content gets transferred to
          reserved area by firmware at the time of
          crash
                   Fig. 1

  o Memory Reservation during second kernel after crash

  Low memory                                        Top of memory
  0      boot memory size                                       |
  |           |<------------- Reserved dump area ----------- -->|
  V           V                                                 V
  +-----------+----------/ /---+---+----+-----------+----+------+
  |           |                |CPU|HPTE|  DUMP     |ELF |      |
  +-----------+----------/ /---+---+----+-----------+----+------+
        |                                              |
        V                                              V
   Used by second                                /proc/vmcore
   kernel to boot
                   Fig. 2

Currently the dump will be copied from /proc/vmcore to a
a new file upon user intervention. The dump data available through
/proc/vmcore will be in ELF format. Hence the existing kdump
infrastructure (kdump scripts) to save the dump works fine with
minor modifications.

The tools to examine the dump will be same as the ones
used for kdump.

How to enable firmware-assisted dump (fadump):
----------------------------------------------

1. Set config option CONFIG_FA_DUMP=y and build kernel.
2. Boot into linux kernel with 'fadump=on' kernel cmdline option.
   By default, fadump reserved memory will be initialized as CMA area.
   Alternatively, user can boot linux kernel with 'fadump=nocma' to
   prevent fadump to use CMA.
3. Optionally, user can also set 'crashkernel=' kernel cmdline
   to specify size of the memory to reserve for boot memory dump
   preservation.

NOTE:
     1. 'fadump_reserve_mem=' parameter has been deprecated. Instead
        use 'crashkernel=' to specify size of the memory to reserve
        for boot memory dump preservation.
     2. If firmware-assisted dump fails to reserve memory then it
        will fallback to existing kdump mechanism if 'crashkernel='
        option is set at kernel cmdline.
     3. if user wants to capture all of user space memory and ok with
        reserved memory not available to production system, then
        'fadump=nocma' kernel parameter can be used to fallback to
        old behaviour.

Sysfs/debugfs files:
--------------------

Firmware-assisted dump feature uses sysfs file system to hold
the control files and debugfs file to display memory reserved region.

Here is the list of files under kernel sysfs:

 /sys/kernel/fadump_enabled
    This is used to display the fadump status.

    - 0 = fadump is disabled
    - 1 = fadump is enabled

    This interface can be used by kdump init scripts to identify if
    fadump is enabled in the kernel and act accordingly.

 /sys/kernel/fadump_registered
    This is used to display the fadump registration status as well
    as to control (start/stop) the fadump registration.

    - 0 = fadump is not registered.
    - 1 = fadump is registered and ready to handle system crash.

    To register fadump echo 1 > /sys/kernel/fadump_registered and
    echo 0 > /sys/kernel/fadump_registered for un-register and stop the
    fadump. Once the fadump is un-registered, the system crash will not
    be handled and vmcore will not be captured. This interface can be
    easily integrated with kdump service start/stop.

 /sys/kernel/fadump_release_mem
    This file is available only when fadump is active during
    second kernel. This is used to release the reserved memory
    region that are held for saving crash dump. To release the
    reserved memory echo 1 to it::

	echo 1  > /sys/kernel/fadump_release_mem

    After echo 1, the content of the /sys/kernel/debug/powerpc/fadump_region
    file will change to reflect the new memory reservations.

    The existing userspace tools (kdump infrastructure) can be easily
    enhanced to use this interface to release the memory reserved for
    dump and continue without 2nd reboot.

Here is the list of files under powerpc debugfs:
(Assuming debugfs is mounted on /sys/kernel/debug directory.)

 /sys/kernel/debug/powerpc/fadump_region
    This file shows the reserved memory regions if fadump is
    enabled otherwise this file is empty. The output format
    is::

      <region>: [<start>-<end>] <reserved-size> bytes, Dumped: <dump-size>

    e.g.
    Contents when fadump is registered during first kernel::

      # cat /sys/kernel/debug/powerpc/fadump_region
      CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x0
      HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x0
      DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x0

    Contents when fadump is active during second kernel::

      # cat /sys/kernel/debug/powerpc/fadump_region
      CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x40020
      HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x1000
      DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x10000000
          : [0x00000010000000-0x0000006ffaffff] 0x5ffb0000 bytes, Dumped: 0x5ffb0000

NOTE:
      Please refer to Documentation/filesystems/debugfs.txt on
      how to mount the debugfs filesystem.


TODO:
-----
 - Need to come up with the better approach to find out more
   accurate boot memory size that is required for a kernel to
   boot successfully when booted with restricted memory.
 - The fadump implementation introduces a fadump crash info structure
   in the scratch area before the ELF core header. The idea of introducing
   this structure is to pass some important crash info data to the second
   kernel which will help second kernel to populate ELF core header with
   correct data before it gets exported through /proc/vmcore. The current
   design implementation does not address a possibility of introducing
   additional fields (in future) to this structure without affecting
   compatibility. Need to come up with the better approach to address this.

   The possible approaches are:

	1. Introduce version field for version tracking, bump up the version
	whenever a new field is added to the structure in future. The version
	field can be used to find out what fields are valid for the current
	version of the structure.
	2. Reserve the area of predefined size (say PAGE_SIZE) for this
	structure and have unused area as reserved (initialized to zero)
	for future field additions.

   The advantage of approach 1 over 2 is we don't need to reserve extra space.

Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>

This document is based on the original documentation written for phyp

assisted dump by Linas Vepstas and Manish Ahuja.