Documentation / powerpc / firmware-assisted-dump.txt

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Based on kernel version 3.9. Page generated on 2013-05-02 23:12 EST.

	
	                   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.
	
	
	TODO:
	-----
	 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 <mahesh@linux.vnet.ibm.com>
	This document is based on the original documentation written for phyp
	assisted dump by Linas Vepstas and Manish Ahuja.

• [ powerpc ]
• 00-INDEX
• bootwrapper.txt
• cpu_features.txt
• eeh-pci-error-recovery.txt
• firmware-assisted-dump.txt
• hvcs.txt
• kvm_440.txt
• mpc52xx.txt
• ptrace.txt
• qe_firmware.txt
• sound.txt
• transactional_memory.txt
• zImage_layout.txt
•