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fadump: Add documentation for firmware-assisted dump.
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 <mahesh@linux.vnet.ibm.com> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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Mahesh Salgaonkar
Benjamin Herrenschmidt
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Documentation/powerpc/firmware-assisted-dump.txt
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Documentation/powerpc/firmware-assisted-dump.txt
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Firmware-Assisted Dump
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------------------------
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July 2011
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The goal of firmware-assisted dump is to enable the dump of
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a crashed system, and to do so from a fully-reset system, and
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to minimize the total elapsed time until the system is back
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in production use.
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- Firmware assisted dump (fadump) infrastructure is intended to replace
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the existing phyp assisted dump.
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- Fadump uses the same firmware interfaces and memory reservation model
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as phyp assisted dump.
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- Unlike phyp dump, fadump exports the memory dump through /proc/vmcore
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in the ELF format in the same way as kdump. This helps us reuse the
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kdump infrastructure for dump capture and filtering.
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- Unlike phyp dump, userspace tool does not need to refer any sysfs
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interface while reading /proc/vmcore.
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- Unlike phyp dump, fadump allows user to release all the memory reserved
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for dump, with a single operation of echo 1 > /sys/kernel/fadump_release_mem.
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- Once enabled through kernel boot parameter, fadump can be
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started/stopped through /sys/kernel/fadump_registered interface (see
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sysfs files section below) and can be easily integrated with kdump
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service start/stop init scripts.
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Comparing with kdump or other strategies, firmware-assisted
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dump offers several strong, practical advantages:
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-- Unlike kdump, the system has been reset, and loaded
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with a fresh copy of the kernel. In particular,
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PCI and I/O devices have been reinitialized and are
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in a clean, consistent state.
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-- Once the dump is copied out, the memory that held the dump
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is immediately available to the running kernel. And therefore,
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unlike kdump, fadump doesn't need a 2nd reboot to get back
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the system to the production configuration.
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The above can only be accomplished by coordination with,
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and assistance from the Power firmware. The procedure is
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as follows:
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-- The first kernel registers the sections of memory with the
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Power firmware for dump preservation during OS initialization.
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These registered sections of memory are reserved by the first
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kernel during early boot.
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-- When a system crashes, the Power firmware will save
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the low memory (boot memory of size larger of 5% of system RAM
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or 256MB) of RAM to the previous registered region. It will
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also save system registers, and hardware PTE's.
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NOTE: The term 'boot memory' means size of the low memory chunk
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that is required for a kernel to boot successfully when
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booted with restricted memory. By default, the boot memory
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size will be the larger of 5% of system RAM or 256MB.
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Alternatively, user can also specify boot memory size
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through boot parameter 'fadump_reserve_mem=' which will
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override the default calculated size. Use this option
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if default boot memory size is not sufficient for second
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kernel to boot successfully.
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-- After the low memory (boot memory) area has been saved, the
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firmware will reset PCI and other hardware state. It will
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*not* clear the RAM. It will then launch the bootloader, as
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normal.
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-- The freshly booted kernel will notice that there is a new
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node (ibm,dump-kernel) in the device tree, indicating that
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there is crash data available from a previous boot. During
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the early boot OS will reserve rest of the memory above
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boot memory size effectively booting with restricted memory
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size. This will make sure that the second kernel will not
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touch any of the dump memory area.
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-- User-space tools will read /proc/vmcore to obtain the contents
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of memory, which holds the previous crashed kernel dump in ELF
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format. The userspace tools may copy this info to disk, or
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network, nas, san, iscsi, etc. as desired.
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-- Once the userspace tool is done saving dump, it will echo
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'1' to /sys/kernel/fadump_release_mem to release the reserved
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memory back to general use, except the memory required for
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next firmware-assisted dump registration.
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e.g.
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# echo 1 > /sys/kernel/fadump_release_mem
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Please note that the firmware-assisted dump feature
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is only available on Power6 and above systems with recent
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firmware versions.
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Implementation details:
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----------------------
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During boot, a check is made to see if firmware supports
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this feature on that particular machine. If it does, then
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we check to see if an active dump is waiting for us. If yes
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then everything but boot memory size of RAM is reserved during
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early boot (See Fig. 2). This area is released once we finish
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collecting the dump from user land scripts (e.g. kdump scripts)
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that are run. If there is dump data, then the
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/sys/kernel/fadump_release_mem file is created, and the reserved
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memory is held.
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If there is no waiting dump data, then only the memory required
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to hold CPU state, HPTE region, boot memory dump and elfcore
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header, is reserved at the top of memory (see Fig. 1). This area
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is *not* released: this region will be kept permanently reserved,
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so that it can act as a receptacle for a copy of the boot memory
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content in addition to CPU state and HPTE region, in the case a
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crash does occur.
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o Memory Reservation during first kernel
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Low memory Top of memory
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0 boot memory size |
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| | |<--Reserved dump area -->|
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V V | Permanent Reservation V
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+-----------+----------/ /----------+---+----+-----------+----+
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| | |CPU|HPTE| DUMP |ELF |
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+-----------+----------/ /----------+---+----+-----------+----+
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| ^
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| |
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\ /
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-------------------------------------------
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Boot memory content gets transferred to
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reserved area by firmware at the time of
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crash
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Fig. 1
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o Memory Reservation during second kernel after crash
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Low memory Top of memory
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0 boot memory size |
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| |<------------- Reserved dump area ----------- -->|
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V V V
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+-----------+----------/ /----------+---+----+-----------+----+
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| | |CPU|HPTE| DUMP |ELF |
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+-----------+----------/ /----------+---+----+-----------+----+
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| |
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V V
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Used by second /proc/vmcore
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kernel to boot
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Fig. 2
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Currently the dump will be copied from /proc/vmcore to a
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a new file upon user intervention. The dump data available through
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/proc/vmcore will be in ELF format. Hence the existing kdump
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infrastructure (kdump scripts) to save the dump works fine with
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minor modifications.
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The tools to examine the dump will be same as the ones
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used for kdump.
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How to enable firmware-assisted dump (fadump):
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-------------------------------------
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1. Set config option CONFIG_FA_DUMP=y and build kernel.
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2. Boot into linux kernel with 'fadump=on' kernel cmdline option.
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3. Optionally, user can also set 'fadump_reserve_mem=' kernel cmdline
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to specify size of the memory to reserve for boot memory dump
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preservation.
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NOTE: If firmware-assisted dump fails to reserve memory then it will
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fallback to existing kdump mechanism if 'crashkernel=' option
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is set at kernel cmdline.
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Sysfs/debugfs files:
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------------
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Firmware-assisted dump feature uses sysfs file system to hold
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the control files and debugfs file to display memory reserved region.
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Here is the list of files under kernel sysfs:
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/sys/kernel/fadump_enabled
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This is used to display the fadump status.
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0 = fadump is disabled
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1 = fadump is enabled
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This interface can be used by kdump init scripts to identify if
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fadump is enabled in the kernel and act accordingly.
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/sys/kernel/fadump_registered
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This is used to display the fadump registration status as well
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as to control (start/stop) the fadump registration.
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0 = fadump is not registered.
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1 = fadump is registered and ready to handle system crash.
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To register fadump echo 1 > /sys/kernel/fadump_registered and
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echo 0 > /sys/kernel/fadump_registered for un-register and stop the
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fadump. Once the fadump is un-registered, the system crash will not
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be handled and vmcore will not be captured. This interface can be
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easily integrated with kdump service start/stop.
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/sys/kernel/fadump_release_mem
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This file is available only when fadump is active during
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second kernel. This is used to release the reserved memory
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region that are held for saving crash dump. To release the
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reserved memory echo 1 to it:
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echo 1 > /sys/kernel/fadump_release_mem
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After echo 1, the content of the /sys/kernel/debug/powerpc/fadump_region
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file will change to reflect the new memory reservations.
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The existing userspace tools (kdump infrastructure) can be easily
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enhanced to use this interface to release the memory reserved for
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dump and continue without 2nd reboot.
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Here is the list of files under powerpc debugfs:
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(Assuming debugfs is mounted on /sys/kernel/debug directory.)
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/sys/kernel/debug/powerpc/fadump_region
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This file shows the reserved memory regions if fadump is
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enabled otherwise this file is empty. The output format
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is:
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<region>: [<start>-<end>] <reserved-size> bytes, Dumped: <dump-size>
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e.g.
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Contents when fadump is registered during first kernel
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# cat /sys/kernel/debug/powerpc/fadump_region
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CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x0
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HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x0
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DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x0
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Contents when fadump is active during second kernel
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# cat /sys/kernel/debug/powerpc/fadump_region
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CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x40020
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HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x1000
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DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x10000000
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: [0x00000010000000-0x0000006ffaffff] 0x5ffb0000 bytes, Dumped: 0x5ffb0000
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NOTE: Please refer to Documentation/filesystems/debugfs.txt on
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how to mount the debugfs filesystem.
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TODO:
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-----
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o Need to come up with the better approach to find out more
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accurate boot memory size that is required for a kernel to
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boot successfully when booted with restricted memory.
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o The fadump implementation introduces a fadump crash info structure
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in the scratch area before the ELF core header. The idea of introducing
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this structure is to pass some important crash info data to the second
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kernel which will help second kernel to populate ELF core header with
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correct data before it gets exported through /proc/vmcore. The current
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design implementation does not address a possibility of introducing
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additional fields (in future) to this structure without affecting
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compatibility. Need to come up with the better approach to address this.
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The possible approaches are:
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1. Introduce version field for version tracking, bump up the version
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whenever a new field is added to the structure in future. The version
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field can be used to find out what fields are valid for the current
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version of the structure.
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2. Reserve the area of predefined size (say PAGE_SIZE) for this
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structure and have unused area as reserved (initialized to zero)
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for future field additions.
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The advantage of approach 1 over 2 is we don't need to reserve extra space.
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---
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Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
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This document is based on the original documentation written for phyp
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assisted dump by Linas Vepstas and Manish Ahuja.
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