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Commit 8468d155 authored by Hari Bathini's avatar Hari Bathini Committed by Michael Ellerman
Browse files

powerpc/fadump: Improve fadump documentation 



The figures depicting FADump's (Firmware-Assisted Dump) memory layout
are missing some finer details like different memory regions and what
they represent. Improve the documentation by updating those details.

Signed-off-by: default avatarHari Bathini <hbathini@linux.ibm.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821322070.5656.8194734198500730487.stgit@hbathini.in.ibm.com
parent 7f0ad11d

Documentation/powerpc/firmware-assisted-dump.rst

+33 −28
Original line number Diff line number Diff line
@@ -76,8 +76,9 @@ as follows: 
   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.

   size. This will make sure that this kernel (also, referred

   to as second kernel or capture 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
@@ -129,41 +130,45 @@ 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

  0      boot memory size      |<--Reserved dump area --->|      |

  |           |                | (Permanent Reservation)  |      |

  V           V                |                          |      V

  +-----------+----------/ /---+---+----+--------+---+----+------+

  |           |                |CPU|HPTE|  DUMP  |HDR|ELF |      |

  +-----------+----------/ /---+---+----+--------+---+----+------+

        |                                   ^      ^

        |                                   |      |

        \                                   /      |

         -----------------------------------     FADump Header

          Boot memory content gets transferred   (meta area)

          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 |      |

  +-----------+----------/ /---+---+----+-----------+----+------+

  |           |<----------- Crash preserved area --------------->|

  V           V                |<-- Reserved dump area -->|      V

  +-----------+----------/ /---+---+----+--------+---+----+------+

  |           |                |CPU|HPTE|  DUMP  |HDR|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.

Currently the dump will be copied from /proc/vmcore to 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. KDump scripts on

major Distro releases have already been modified to work seemlessly (no

user intervention in saving the dump) when FADump is used, instead of

KDump, as dump mechanism.



The tools to examine the dump will be same as the ones

used for kdump.