1================================================================
2Documentation for Kdump - The kexec-based Crash Dumping Solution
3================================================================
4
5This document includes overview, setup, installation, and analysis
6information.
7
8Overview
9========
10
11Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12dump of the system kernel's memory needs to be taken (for example, when
13the system panics). The system kernel's memory image is preserved across
14the reboot and is accessible to the dump-capture kernel.
15
16You can use common commands, such as cp, scp or makedumpfile to copy
17the memory image to a dump file on the local disk, or across the network
18to a remote system.
19
20Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
21s390x, arm and arm64 architectures.
22
23When the system kernel boots, it reserves a small section of memory for
24the dump-capture kernel. This ensures that ongoing Direct Memory Access
25(DMA) from the system kernel does not corrupt the dump-capture kernel.
26The kexec -p command loads the dump-capture kernel into this reserved
27memory.
28
29On x86 machines, the first 640 KB of physical memory is needed for boot,
30regardless of where the kernel loads. For simpler handling, the whole
31low 1M is reserved to avoid any later kernel or device driver writing
32data into this area. Like this, the low 1M can be reused as system RAM
33by kdump kernel without extra handling.
34
35On PPC64 machines first 32KB of physical memory is needed for booting
36regardless of where the kernel is loaded and to support 64K page size
37kexec backs up the first 64KB memory.
38
39For s390x, when kdump is triggered, the crashkernel region is exchanged
40with the region [0, crashkernel region size] and then the kdump kernel
41runs in [0, crashkernel region size]. Therefore no relocatable kernel is
42needed for s390x.
43
44All of the necessary information about the system kernel's core image is
45encoded in the ELF format, and stored in a reserved area of memory
46before a crash. The physical address of the start of the ELF header is
47passed to the dump-capture kernel through the elfcorehdr= boot
48parameter. Optionally the size of the ELF header can also be passed
49when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
50
51With the dump-capture kernel, you can access the memory image through
52/proc/vmcore. This exports the dump as an ELF-format file that you can
53write out using file copy commands such as cp or scp. You can also use
54makedumpfile utility to analyze and write out filtered contents with
55options, e.g with '-d 31' it will only write out kernel data. Further,
56you can use analysis tools such as the GNU Debugger (GDB) and the Crash
57tool to debug the dump file. This method ensures that the dump pages are
58correctly ordered.
59
60Setup and Installation
61======================
62
63Install kexec-tools
64-------------------
65
661) Login as the root user.
67
682) Download the kexec-tools user-space package from the following URL:
69
70http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
71
72This is a symlink to the latest version.
73
74The latest kexec-tools git tree is available at:
75
76- git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
77- http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
78
79There is also a gitweb interface available at
80http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
81
82More information about kexec-tools can be found at
83http://horms.net/projects/kexec/
84
853) Unpack the tarball with the tar command, as follows::
86
87	tar xvpzf kexec-tools.tar.gz
88
894) Change to the kexec-tools directory, as follows::
90
91	cd kexec-tools-VERSION
92
935) Configure the package, as follows::
94
95	./configure
96
976) Compile the package, as follows::
98
99	make
100
1017) Install the package, as follows::
102
103	make install
104
105
106Build the system and dump-capture kernels
107-----------------------------------------
108There are two possible methods of using Kdump.
109
1101) Build a separate custom dump-capture kernel for capturing the
111   kernel core dump.
112
1132) Or use the system kernel binary itself as dump-capture kernel and there is
114   no need to build a separate dump-capture kernel. This is possible
115   only with the architectures which support a relocatable kernel. As
116   of today, i386, x86_64, ppc64, ia64, arm and arm64 architectures support
117   relocatable kernel.
118
119Building a relocatable kernel is advantageous from the point of view that
120one does not have to build a second kernel for capturing the dump. But
121at the same time one might want to build a custom dump capture kernel
122suitable to his needs.
123
124Following are the configuration setting required for system and
125dump-capture kernels for enabling kdump support.
126
127System kernel config options
128----------------------------
129
1301) Enable "kexec system call" or "kexec file based system call" in
131   "Processor type and features."::
132
133	CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y
134
135   And both of them will select KEXEC_CORE::
136
137	CONFIG_KEXEC_CORE=y
138
139   Subsequently, CRASH_CORE is selected by KEXEC_CORE::
140
141	CONFIG_CRASH_CORE=y
142
1432) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
144   filesystems." This is usually enabled by default::
145
146	CONFIG_SYSFS=y
147
148   Note that "sysfs file system support" might not appear in the "Pseudo
149   filesystems" menu if "Configure standard kernel features (for small
150   systems)" is not enabled in "General Setup." In this case, check the
151   .config file itself to ensure that sysfs is turned on, as follows::
152
153	grep 'CONFIG_SYSFS' .config
154
1553) Enable "Compile the kernel with debug info" in "Kernel hacking."::
156
157	CONFIG_DEBUG_INFO=Y
158
159   This causes the kernel to be built with debug symbols. The dump
160   analysis tools require a vmlinux with debug symbols in order to read
161   and analyze a dump file.
162
163Dump-capture kernel config options (Arch Independent)
164-----------------------------------------------------
165
1661) Enable "kernel crash dumps" support under "Processor type and
167   features"::
168
169	CONFIG_CRASH_DUMP=y
170
1712) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems"::
172
173	CONFIG_PROC_VMCORE=y
174
175   (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
176
177Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
178--------------------------------------------------------------------
179
1801) On i386, enable high memory support under "Processor type and
181   features"::
182
183	CONFIG_HIGHMEM64G=y
184
185   or::
186
187	CONFIG_HIGHMEM4G
188
1892) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel
190   command line when loading the dump-capture kernel because one
191   CPU is enough for kdump kernel to dump vmcore on most of systems.
192
193   However, you can also specify nr_cpus=X to enable multiple processors
194   in kdump kernel. In this case, "disable_cpu_apicid=" is needed to
195   tell kdump kernel which cpu is 1st kernel's BSP. Please refer to
196   admin-guide/kernel-parameters.txt for more details.
197
198   With CONFIG_SMP=n, the above things are not related.
199
2003) A relocatable kernel is suggested to be built by default. If not yet,
201   enable "Build a relocatable kernel" support under "Processor type and
202   features"::
203
204	CONFIG_RELOCATABLE=y
205
2064) Use a suitable value for "Physical address where the kernel is
207   loaded" (under "Processor type and features"). This only appears when
208   "kernel crash dumps" is enabled. A suitable value depends upon
209   whether kernel is relocatable or not.
210
211   If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
212   This will compile the kernel for physical address 1MB, but given the fact
213   kernel is relocatable, it can be run from any physical address hence
214   kexec boot loader will load it in memory region reserved for dump-capture
215   kernel.
216
217   Otherwise it should be the start of memory region reserved for
218   second kernel using boot parameter "crashkernel=Y@X". Here X is
219   start of memory region reserved for dump-capture kernel.
220   Generally X is 16MB (0x1000000). So you can set
221   CONFIG_PHYSICAL_START=0x1000000
222
2235) Make and install the kernel and its modules. DO NOT add this kernel
224   to the boot loader configuration files.
225
226Dump-capture kernel config options (Arch Dependent, ppc64)
227----------------------------------------------------------
228
2291) Enable "Build a kdump crash kernel" support under "Kernel" options::
230
231	CONFIG_CRASH_DUMP=y
232
2332)   Enable "Build a relocatable kernel" support::
234
235	CONFIG_RELOCATABLE=y
236
237   Make and install the kernel and its modules.
238
239Dump-capture kernel config options (Arch Dependent, ia64)
240----------------------------------------------------------
241
242- No specific options are required to create a dump-capture kernel
243  for ia64, other than those specified in the arch independent section
244  above. This means that it is possible to use the system kernel
245  as a dump-capture kernel if desired.
246
247  The crashkernel region can be automatically placed by the system
248  kernel at runtime. This is done by specifying the base address as 0,
249  or omitting it all together::
250
251	crashkernel=256M@0
252
253  or::
254
255	crashkernel=256M
256
257Dump-capture kernel config options (Arch Dependent, arm)
258----------------------------------------------------------
259
260-   To use a relocatable kernel,
261    Enable "AUTO_ZRELADDR" support under "Boot" options::
262
263	AUTO_ZRELADDR=y
264
265Dump-capture kernel config options (Arch Dependent, arm64)
266----------------------------------------------------------
267
268- Please note that kvm of the dump-capture kernel will not be enabled
269  on non-VHE systems even if it is configured. This is because the CPU
270  will not be reset to EL2 on panic.
271
272crashkernel syntax
273===========================
2741) crashkernel=size@offset
275
276   Here 'size' specifies how much memory to reserve for the dump-capture kernel
277   and 'offset' specifies the beginning of this reserved memory. For example,
278   "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
279   starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
280
281   The crashkernel region can be automatically placed by the system
282   kernel at run time. This is done by specifying the base address as 0,
283   or omitting it all together::
284
285         crashkernel=256M@0
286
287   or::
288
289         crashkernel=256M
290
291   If the start address is specified, note that the start address of the
292   kernel will be aligned to a value (which is Arch dependent), so if the
293   start address is not then any space below the alignment point will be
294   wasted.
295
2962) range1:size1[,range2:size2,...][@offset]
297
298   While the "crashkernel=size[@offset]" syntax is sufficient for most
299   configurations, sometimes it's handy to have the reserved memory dependent
300   on the value of System RAM -- that's mostly for distributors that pre-setup
301   the kernel command line to avoid a unbootable system after some memory has
302   been removed from the machine.
303
304   The syntax is::
305
306       crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
307       range=start-[end]
308
309   For example::
310
311       crashkernel=512M-2G:64M,2G-:128M
312
313   This would mean:
314
315       1) if the RAM is smaller than 512M, then don't reserve anything
316          (this is the "rescue" case)
317       2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
318       3) if the RAM size is larger than 2G, then reserve 128M
319
3203) crashkernel=size,high and crashkernel=size,low
321
322   If memory above 4G is preferred, crashkernel=size,high can be used to
323   fulfill that. With it, physical memory is allowed to be allocated from top,
324   so could be above 4G if system has more than 4G RAM installed. Otherwise,
325   memory region will be allocated below 4G if available.
326
327   When crashkernel=X,high is passed, kernel could allocate physical memory
328   region above 4G, low memory under 4G is needed in this case. There are
329   three ways to get low memory:
330
331      1) Kernel will allocate at least 256M memory below 4G automatically
332         if crashkernel=Y,low is not specified.
333      2) Let user specify low memory size instead.
334      3) Specified value 0 will disable low memory allocation::
335
336            crashkernel=0,low
337
338Boot into System Kernel
339-----------------------
3401) Update the boot loader (such as grub, yaboot, or lilo) configuration
341   files as necessary.
342
3432) Boot the system kernel with the boot parameter "crashkernel=Y@X".
344
345   On x86 and x86_64, use "crashkernel=Y[@X]". Most of the time, the
346   start address 'X' is not necessary, kernel will search a suitable
347   area. Unless an explicit start address is expected.
348
349   On ppc64, use "crashkernel=128M@32M".
350
351   On ia64, 256M@256M is a generous value that typically works.
352   The region may be automatically placed on ia64, see the
353   dump-capture kernel config option notes above.
354   If use sparse memory, the size should be rounded to GRANULE boundaries.
355
356   On s390x, typically use "crashkernel=xxM". The value of xx is dependent
357   on the memory consumption of the kdump system. In general this is not
358   dependent on the memory size of the production system.
359
360   On arm, the use of "crashkernel=Y@X" is no longer necessary; the
361   kernel will automatically locate the crash kernel image within the
362   first 512MB of RAM if X is not given.
363
364   On arm64, use "crashkernel=Y[@X]".  Note that the start address of
365   the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
366
367Load the Dump-capture Kernel
368============================
369
370After booting to the system kernel, dump-capture kernel needs to be
371loaded.
372
373Based on the architecture and type of image (relocatable or not), one
374can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
375of dump-capture kernel. Following is the summary.
376
377For i386 and x86_64:
378
379	- Use bzImage/vmlinuz if kernel is relocatable.
380	- Use vmlinux if kernel is not relocatable.
381
382For ppc64:
383
384	- Use vmlinux
385
386For ia64:
387
388	- Use vmlinux or vmlinuz.gz
389
390For s390x:
391
392	- Use image or bzImage
393
394For arm:
395
396	- Use zImage
397
398For arm64:
399
400	- Use vmlinux or Image
401
402If you are using an uncompressed vmlinux image then use following command
403to load dump-capture kernel::
404
405   kexec -p <dump-capture-kernel-vmlinux-image> \
406   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
407   --append="root=<root-dev> <arch-specific-options>"
408
409If you are using a compressed bzImage/vmlinuz, then use following command
410to load dump-capture kernel::
411
412   kexec -p <dump-capture-kernel-bzImage> \
413   --initrd=<initrd-for-dump-capture-kernel> \
414   --append="root=<root-dev> <arch-specific-options>"
415
416If you are using a compressed zImage, then use following command
417to load dump-capture kernel::
418
419   kexec --type zImage -p <dump-capture-kernel-bzImage> \
420   --initrd=<initrd-for-dump-capture-kernel> \
421   --dtb=<dtb-for-dump-capture-kernel> \
422   --append="root=<root-dev> <arch-specific-options>"
423
424If you are using an uncompressed Image, then use following command
425to load dump-capture kernel::
426
427   kexec -p <dump-capture-kernel-Image> \
428   --initrd=<initrd-for-dump-capture-kernel> \
429   --append="root=<root-dev> <arch-specific-options>"
430
431Please note, that --args-linux does not need to be specified for ia64.
432It is planned to make this a no-op on that architecture, but for now
433it should be omitted
434
435Following are the arch specific command line options to be used while
436loading dump-capture kernel.
437
438For i386, x86_64 and ia64:
439
440	"1 irqpoll nr_cpus=1 reset_devices"
441
442For ppc64:
443
444	"1 maxcpus=1 noirqdistrib reset_devices"
445
446For s390x:
447
448	"1 nr_cpus=1 cgroup_disable=memory"
449
450For arm:
451
452	"1 maxcpus=1 reset_devices"
453
454For arm64:
455
456	"1 nr_cpus=1 reset_devices"
457
458Notes on loading the dump-capture kernel:
459
460* By default, the ELF headers are stored in ELF64 format to support
461  systems with more than 4GB memory. On i386, kexec automatically checks if
462  the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
463  So, on non-PAE systems, ELF32 is always used.
464
465  The --elf32-core-headers option can be used to force the generation of ELF32
466  headers. This is necessary because GDB currently cannot open vmcore files
467  with ELF64 headers on 32-bit systems.
468
469* The "irqpoll" boot parameter reduces driver initialization failures
470  due to shared interrupts in the dump-capture kernel.
471
472* You must specify <root-dev> in the format corresponding to the root
473  device name in the output of mount command.
474
475* Boot parameter "1" boots the dump-capture kernel into single-user
476  mode without networking. If you want networking, use "3".
477
478* We generally don't have to bring up a SMP kernel just to capture the
479  dump. Hence generally it is useful either to build a UP dump-capture
480  kernel or specify maxcpus=1 option while loading dump-capture kernel.
481  Note, though maxcpus always works, you had better replace it with
482  nr_cpus to save memory if supported by the current ARCH, such as x86.
483
484* You should enable multi-cpu support in dump-capture kernel if you intend
485  to use multi-thread programs with it, such as parallel dump feature of
486  makedumpfile. Otherwise, the multi-thread program may have a great
487  performance degradation. To enable multi-cpu support, you should bring up an
488  SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
489  options while loading it.
490
491* For s390x there are two kdump modes: If a ELF header is specified with
492  the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
493  is done on all other architectures. If no elfcorehdr= kernel parameter is
494  specified, the s390x kdump kernel dynamically creates the header. The
495  second mode has the advantage that for CPU and memory hotplug, kdump has
496  not to be reloaded with kexec_load().
497
498* For s390x systems with many attached devices the "cio_ignore" kernel
499  parameter should be used for the kdump kernel in order to prevent allocation
500  of kernel memory for devices that are not relevant for kdump. The same
501  applies to systems that use SCSI/FCP devices. In that case the
502  "allow_lun_scan" zfcp module parameter should be set to zero before
503  setting FCP devices online.
504
505Kernel Panic
506============
507
508After successfully loading the dump-capture kernel as previously
509described, the system will reboot into the dump-capture kernel if a
510system crash is triggered.  Trigger points are located in panic(),
511die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
512
513The following conditions will execute a crash trigger point:
514
515If a hard lockup is detected and "NMI watchdog" is configured, the system
516will boot into the dump-capture kernel ( die_nmi() ).
517
518If die() is called, and it happens to be a thread with pid 0 or 1, or die()
519is called inside interrupt context or die() is called and panic_on_oops is set,
520the system will boot into the dump-capture kernel.
521
522On powerpc systems when a soft-reset is generated, die() is called by all cpus
523and the system will boot into the dump-capture kernel.
524
525For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
526"echo c > /proc/sysrq-trigger" or write a module to force the panic.
527
528Write Out the Dump File
529=======================
530
531After the dump-capture kernel is booted, write out the dump file with
532the following command::
533
534   cp /proc/vmcore <dump-file>
535
536You can also use makedumpfile utility to write out the dump file
537with specified options to filter out unwanted contents, e.g::
538
539   makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>
540
541Analysis
542========
543
544Before analyzing the dump image, you should reboot into a stable kernel.
545
546You can do limited analysis using GDB on the dump file copied out of
547/proc/vmcore. Use the debug vmlinux built with -g and run the following
548command::
549
550   gdb vmlinux <dump-file>
551
552Stack trace for the task on processor 0, register display, and memory
553display work fine.
554
555Note: GDB cannot analyze core files generated in ELF64 format for x86.
556On systems with a maximum of 4GB of memory, you can generate
557ELF32-format headers using the --elf32-core-headers kernel option on the
558dump kernel.
559
560You can also use the Crash utility to analyze dump files in Kdump
561format. Crash is available at the following URL:
562
563   https://github.com/crash-utility/crash
564
565Crash document can be found at:
566   https://crash-utility.github.io/
567
568Trigger Kdump on WARN()
569=======================
570
571The kernel parameter, panic_on_warn, calls panic() in all WARN() paths.  This
572will cause a kdump to occur at the panic() call.  In cases where a user wants
573to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
574to achieve the same behaviour.
575
576Trigger Kdump on add_taint()
577============================
578
579The kernel parameter panic_on_taint facilitates a conditional call to panic()
580from within add_taint() whenever the value set in this bitmask matches with the
581bit flag being set by add_taint().
582This will cause a kdump to occur at the add_taint()->panic() call.
583
584Contact
585=======
586
587- kexec@lists.infradead.org
588
589GDB macros
590==========
591
592.. include:: gdbmacros.txt
593   :literal:
594