362 lines
16 KiB
ReStructuredText
362 lines
16 KiB
ReStructuredText
======================
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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 system crashes, the Power firmware will copy the registered
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low memory regions (boot memory) from source to destination area.
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It will also save hardware PTE's.
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NOTE:
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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 'crashkernel=' which will override
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the default calculated size. Use this option if default
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boot memory size is not sufficient for second kernel to
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boot successfully. For syntax of crashkernel= parameter,
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refer to Documentation/admin-guide/kdump/kdump.rst. If any
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offset is provided in crashkernel= parameter, it will be
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ignored as FADump uses a predefined offset to reserve memory
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for boot memory dump preservation in case of a crash.
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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 node
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(rtas/ibm,kernel-dump on pSeries or ibm,opal/dump/mpipl-boot
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on OPAL platform) 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 this kernel (also, referred
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to as second kernel or capture kernel) will not touch any
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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 on pSeries
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(PowerVM) platform and POWER9 and above systems with OP940
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or later firmware versions on PowerNV (OPAL) platform.
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Note that, OPAL firmware exports ibm,opal/dump node when
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FADump is supported on PowerNV platform.
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On OPAL based machines, system first boots into an intermittent
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kernel (referred to as petitboot kernel) before booting into the
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capture kernel. This kernel would have minimal kernel and/or
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userspace support to process crash data. Such kernel needs to
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preserve previously crash'ed kernel's memory for the subsequent
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capture kernel boot to process this crash data. Kernel config
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option CONFIG_PRESERVE_FA_DUMP has to be enabled on such kernel
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to ensure that crash data is preserved to process later.
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-- On OPAL based machines (PowerNV), if the kernel is build with
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CONFIG_OPAL_CORE=y, OPAL memory at the time of crash is also
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exported as /sys/firmware/opal/core file. This procfs file is
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helpful in debugging OPAL crashes with GDB. The kernel memory
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used for exporting this procfs file can be released by echo'ing
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'1' to /sys/kernel/fadump_release_opalcore node.
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e.g.
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# echo 1 > /sys/kernel/fadump_release_opalcore
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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 to
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hold CPU state, HPTE region, boot memory dump, FADump header and
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elfcore header, is usually reserved at an offset greater than boot
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memory size (see Fig. 1). This area is *not* released: this region
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will be kept permanently reserved, so that it can act as a receptacle
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for a copy of the boot memory content in addition to CPU state and
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HPTE region, in the case a crash does occur.
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Since this reserved memory area is used only after the system crash,
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there is no point in blocking this significant chunk of memory from
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production kernel. Hence, the implementation uses the Linux kernel's
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Contiguous Memory Allocator (CMA) for memory reservation if CMA is
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configured for kernel. With CMA reservation this memory will be
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available for applications to use it, while kernel is prevented from
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using it. With this FADump will still be able to capture all of the
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kernel memory and most of the user space memory except the user pages
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that were present in CMA region::
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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 |<--- Reserved dump area --->| |
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| | | Permanent Reservation | |
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V V | | V
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+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
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| | |///|////| DUMP | HDR | ELF |////| |
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+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
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| ^ ^ ^ ^ ^
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| | | | | |
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\ CPU HPTE / | |
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------------------------------ | |
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Boot memory content gets transferred | |
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to reserved area by firmware at the | |
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time of crash. | |
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FADump Header |
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(meta area) |
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Metadata: This area holds a metadata struture whose
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address is registered with f/w and retrieved in the
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second kernel after crash, on platforms that support
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tags (OPAL). Having such structure with info needed
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to process the crashdump eases dump capture process.
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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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| |<------------ Crash preserved area ------------>|
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V V |<--- Reserved dump area --->| |
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+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
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| | |///|////| DUMP | HDR | ELF |////| |
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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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+---+
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|///| -> Regions (CPU, HPTE & Metadata) marked like this in the above
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+---+ figures are not always present. For example, OPAL platform
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does not have CPU & HPTE regions while Metadata region is
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not supported on pSeries currently.
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Fig. 2
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Currently the dump will be copied from /proc/vmcore to a new file upon
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user intervention. The dump data available through /proc/vmcore will be
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in ELF format. Hence the existing kdump infrastructure (kdump scripts)
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to save the dump works fine with minor modifications. KDump scripts on
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major Distro releases have already been modified to work seemlessly (no
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user intervention in saving the dump) when FADump is used, instead of
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KDump, as dump mechanism.
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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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By default, FADump reserved memory will be initialized as CMA area.
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Alternatively, user can boot linux kernel with 'fadump=nocma' to
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prevent FADump to use CMA.
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3. Optionally, user can also set 'crashkernel=' 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:
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1. 'fadump_reserve_mem=' parameter has been deprecated. Instead
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use 'crashkernel=' to specify size of the memory to reserve
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for boot memory dump preservation.
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2. If firmware-assisted dump fails to reserve memory then it
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will fallback to existing kdump mechanism if 'crashkernel='
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option is set at kernel cmdline.
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3. if user wants to capture all of user space memory and ok with
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reserved memory not available to production system, then
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'fadump=nocma' kernel parameter can be used to fallback to
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old behaviour.
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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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/sys/kernel/fadump_release_opalcore
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This file is available only on OPAL based machines when FADump is
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active during capture kernel. This is used to release the memory
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used by the kernel to export /sys/firmware/opal/core file. To
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release this memory, echo '1' to it:
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echo 1 > /sys/kernel/fadump_release_opalcore
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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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and for kernel DUMP region is:
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DUMP: Src: <src-addr>, Dest: <dest-addr>, Size: <size>, Dumped: # bytes
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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:
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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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- 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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- 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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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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