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    Subject[RFC PATCH v7 01/10] fadump: Add documentation for firmware-assisted dump.
    From: Mahesh Salgaonkar <>

    Documentation for firmware-assisted dump. This document is based on the
    original documentation written for phyp assisted dump by Linas Vepstas
    and Manish Ahuja, with few changes to reflect the current implementation.

    Signed-off-by: Mahesh Salgaonkar <>
    Documentation/powerpc/firmware-assisted-dump.txt | 270 ++++++++++++++++++++++
    1 files changed, 270 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/powerpc/firmware-assisted-dump.txt

    diff --git a/Documentation/powerpc/firmware-assisted-dump.txt b/Documentation/powerpc/firmware-assisted-dump.txt
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    + 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 'fadump_reserve_mem=' which will
    + override the default calculated size. Use this option
    + if default boot memory size is not sufficient for second
    + kernel to boot successfully.
    +-- 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 reserved at the top of memory (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.
    + 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.
    +3. Optionally, user can also set 'fadump_reserve_mem=' kernel cmdline
    + to specify size of the memory to reserve for boot memory dump
    + preservation.
    +NOTE: If firmware-assisted dump fails to reserve memory then it will
    + fallback to existing kdump mechanism if 'crashkernel=' option
    + is set at kernel cmdline.
    +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.
    + o 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.
    + o 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 <>
    +This document is based on the original documentation written for phyp
    +assisted dump by Linas Vepstas and Manish Ahuja.

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