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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 ++++++++++++++++++++++
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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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