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-================================================================
-Documentation for Kdump - The kexec-based Crash Dumping Solution
-================================================================
-
-This document includes overview, setup and installation, and analysis
-information.
-
-Overview
-========
-
-Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
-dump of the system kernel's memory needs to be taken (for example, when
-the system panics). The system kernel's memory image is preserved across
-the reboot and is accessible to the dump-capture kernel.
-
-You can use common commands, such as cp and scp, to copy the
-memory image to a dump file on the local disk, or across the network to
-a remote system.
-
-Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
-and s390x architectures.
-
-When the system kernel boots, it reserves a small section of memory for
-the dump-capture kernel. This ensures that ongoing Direct Memory Access
-(DMA) from the system kernel does not corrupt the dump-capture kernel.
-The kexec -p command loads the dump-capture kernel into this reserved
-memory.
-
-On x86 machines, the first 640 KB of physical memory is needed to boot,
-regardless of where the kernel loads. Therefore, kexec backs up this
-region just before rebooting into the dump-capture kernel.
-
-Similarly on PPC64 machines first 32KB of physical memory is needed for
-booting regardless of where the kernel is loaded and to support 64K page
-size kexec backs up the first 64KB memory.
-
-For s390x, when kdump is triggered, the crashkernel region is exchanged
-with the region [0, crashkernel region size] and then the kdump kernel
-runs in [0, crashkernel region size]. Therefore no relocatable kernel is
-needed for s390x.
-
-All of the necessary information about the system kernel's core image is
-encoded in the ELF format, and stored in a reserved area of memory
-before a crash. The physical address of the start of the ELF header is
-passed to the dump-capture kernel through the elfcorehdr= boot
-parameter. Optionally the size of the ELF header can also be passed
-when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
-
-
-With the dump-capture kernel, you can access the memory image, or "old
-memory," in two ways:
-
-- Through a /dev/oldmem device interface. A capture utility can read the
- device file and write out the memory in raw format. This is a raw dump
- of memory. Analysis and capture tools must be intelligent enough to
- determine where to look for the right information.
-
-- Through /proc/vmcore. This exports the dump as an ELF-format file that
- you can write out using file copy commands such as cp or scp. Further,
- you can use analysis tools such as the GNU Debugger (GDB) and the Crash
- tool to debug the dump file. This method ensures that the dump pages are
- correctly ordered.
-
-
-Setup and Installation
-======================
-
-Install kexec-tools
--------------------
-
-1) Login as the root user.
-
-2) Download the kexec-tools user-space package from the following URL:
-
-http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
-
-This is a symlink to the latest version.
-
-The latest kexec-tools git tree is available at:
-
-git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
-and
-http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
-
-There is also a gitweb interface available at
-http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
-
-More information about kexec-tools can be found at
-http://www.kernel.org/pub/linux/utils/kernel/kexec/README.html
-
-3) Unpack the tarball with the tar command, as follows:
-
- tar xvpzf kexec-tools.tar.gz
-
-4) Change to the kexec-tools directory, as follows:
-
- cd kexec-tools-VERSION
-
-5) Configure the package, as follows:
-
- ./configure
-
-6) Compile the package, as follows:
-
- make
-
-7) Install the package, as follows:
-
- make install
-
-
-Build the system and dump-capture kernels
------------------------------------------
-There are two possible methods of using Kdump.
-
-1) Build a separate custom dump-capture kernel for capturing the
- kernel core dump.
-
-2) Or use the system kernel binary itself as dump-capture kernel and there is
- no need to build a separate dump-capture kernel. This is possible
- only with the architectures which support a relocatable kernel. As
- of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
- kernel.
-
-Building a relocatable kernel is advantageous from the point of view that
-one does not have to build a second kernel for capturing the dump. But
-at the same time one might want to build a custom dump capture kernel
-suitable to his needs.
-
-Following are the configuration setting required for system and
-dump-capture kernels for enabling kdump support.
-
-System kernel config options
-----------------------------
-
-1) Enable "kexec system call" in "Processor type and features."
-
- CONFIG_KEXEC=y
-
-2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
- filesystems." This is usually enabled by default.
-
- CONFIG_SYSFS=y
-
- Note that "sysfs file system support" might not appear in the "Pseudo
- filesystems" menu if "Configure standard kernel features (for small
- systems)" is not enabled in "General Setup." In this case, check the
- .config file itself to ensure that sysfs is turned on, as follows:
-
- grep 'CONFIG_SYSFS' .config
-
-3) Enable "Compile the kernel with debug info" in "Kernel hacking."
-
- CONFIG_DEBUG_INFO=Y
-
- This causes the kernel to be built with debug symbols. The dump
- analysis tools require a vmlinux with debug symbols in order to read
- and analyze a dump file.
-
-Dump-capture kernel config options (Arch Independent)
------------------------------------------------------
-
-1) Enable "kernel crash dumps" support under "Processor type and
- features":
-
- CONFIG_CRASH_DUMP=y
-
-2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems".
-
- CONFIG_PROC_VMCORE=y
- (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
-
-Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
---------------------------------------------------------------------
-
-1) On i386, enable high memory support under "Processor type and
- features":
-
- CONFIG_HIGHMEM64G=y
- or
- CONFIG_HIGHMEM4G
-
-2) On i386 and x86_64, disable symmetric multi-processing support
- under "Processor type and features":
-
- CONFIG_SMP=n
-
- (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
- when loading the dump-capture kernel, see section "Load the Dump-capture
- Kernel".)
-
-3) If one wants to build and use a relocatable kernel,
- Enable "Build a relocatable kernel" support under "Processor type and
- features"
-
- CONFIG_RELOCATABLE=y
-
-4) Use a suitable value for "Physical address where the kernel is
- loaded" (under "Processor type and features"). This only appears when
- "kernel crash dumps" is enabled. A suitable value depends upon
- whether kernel is relocatable or not.
-
- If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
- This will compile the kernel for physical address 1MB, but given the fact
- kernel is relocatable, it can be run from any physical address hence
- kexec boot loader will load it in memory region reserved for dump-capture
- kernel.
-
- Otherwise it should be the start of memory region reserved for
- second kernel using boot parameter "crashkernel=Y@X". Here X is
- start of memory region reserved for dump-capture kernel.
- Generally X is 16MB (0x1000000). So you can set
- CONFIG_PHYSICAL_START=0x1000000
-
-5) Make and install the kernel and its modules. DO NOT add this kernel
- to the boot loader configuration files.
-
-Dump-capture kernel config options (Arch Dependent, ppc64)
-----------------------------------------------------------
-
-1) Enable "Build a kdump crash kernel" support under "Kernel" options:
-
- CONFIG_CRASH_DUMP=y
-
-2) Enable "Build a relocatable kernel" support
-
- CONFIG_RELOCATABLE=y
-
- Make and install the kernel and its modules.
-
-Dump-capture kernel config options (Arch Dependent, ia64)
-----------------------------------------------------------
-
-- No specific options are required to create a dump-capture kernel
- for ia64, other than those specified in the arch independent section
- above. This means that it is possible to use the system kernel
- as a dump-capture kernel if desired.
-
- The crashkernel region can be automatically placed by the system
- kernel at run time. This is done by specifying the base address as 0,
- or omitting it all together.
-
- crashkernel=256M@0
- or
- crashkernel=256M
-
- If the start address is specified, note that the start address of the
- kernel will be aligned to 64Mb, so if the start address is not then
- any space below the alignment point will be wasted.
-
-
-Extended crashkernel syntax
-===========================
-
-While the "crashkernel=size[@offset]" syntax is sufficient for most
-configurations, sometimes it's handy to have the reserved memory dependent
-on the value of System RAM -- that's mostly for distributors that pre-setup
-the kernel command line to avoid a unbootable system after some memory has
-been removed from the machine.
-
-The syntax is:
-
- crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
- range=start-[end]
-
- 'start' is inclusive and 'end' is exclusive.
-
-For example:
-
- crashkernel=512M-2G:64M,2G-:128M
-
-This would mean:
-
- 1) if the RAM is smaller than 512M, then don't reserve anything
- (this is the "rescue" case)
- 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
- 3) if the RAM size is larger than 2G, then reserve 128M
-
-
-
-Boot into System Kernel
-=======================
-
-1) Update the boot loader (such as grub, yaboot, or lilo) configuration
- files as necessary.
-
-2) Boot the system kernel with the boot parameter "crashkernel=Y@X",
- where Y specifies how much memory to reserve for the dump-capture kernel
- and X specifies the beginning of this reserved memory. For example,
- "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
- starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
-
- On x86 and x86_64, use "crashkernel=64M@16M".
-
- On ppc64, use "crashkernel=128M@32M".
-
- On ia64, 256M@256M is a generous value that typically works.
- The region may be automatically placed on ia64, see the
- dump-capture kernel config option notes above.
-
- On s390x, typically use "crashkernel=xxM". The value of xx is dependent
- on the memory consumption of the kdump system. In general this is not
- dependent on the memory size of the production system.
-
-Load the Dump-capture Kernel
-============================
-
-After booting to the system kernel, dump-capture kernel needs to be
-loaded.
-
-Based on the architecture and type of image (relocatable or not), one
-can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
-of dump-capture kernel. Following is the summary.
-
-For i386 and x86_64:
- - Use vmlinux if kernel is not relocatable.
- - Use bzImage/vmlinuz if kernel is relocatable.
-For ppc64:
- - Use vmlinux
-For ia64:
- - Use vmlinux or vmlinuz.gz
-For s390x:
- - Use image or bzImage
-
-
-If you are using a uncompressed vmlinux image then use following command
-to load dump-capture kernel.
-
- kexec -p <dump-capture-kernel-vmlinux-image> \
- --initrd=<initrd-for-dump-capture-kernel> --args-linux \
- --append="root=<root-dev> <arch-specific-options>"
-
-If you are using a compressed bzImage/vmlinuz, then use following command
-to load dump-capture kernel.
-
- kexec -p <dump-capture-kernel-bzImage> \
- --initrd=<initrd-for-dump-capture-kernel> \
- --append="root=<root-dev> <arch-specific-options>"
-
-Please note, that --args-linux does not need to be specified for ia64.
-It is planned to make this a no-op on that architecture, but for now
-it should be omitted
-
-Following are the arch specific command line options to be used while
-loading dump-capture kernel.
-
-For i386, x86_64 and ia64:
- "1 irqpoll maxcpus=1 reset_devices"
-
-For ppc64:
- "1 maxcpus=1 noirqdistrib reset_devices"
-
-For s390x:
- "1 maxcpus=1 cgroup_disable=memory"
-
-Notes on loading the dump-capture kernel:
-
-* By default, the ELF headers are stored in ELF64 format to support
- systems with more than 4GB memory. On i386, kexec automatically checks if
- the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
- So, on non-PAE systems, ELF32 is always used.
-
- The --elf32-core-headers option can be used to force the generation of ELF32
- headers. This is necessary because GDB currently cannot open vmcore files
- with ELF64 headers on 32-bit systems.
-
-* The "irqpoll" boot parameter reduces driver initialization failures
- due to shared interrupts in the dump-capture kernel.
-
-* You must specify <root-dev> in the format corresponding to the root
- device name in the output of mount command.
-
-* Boot parameter "1" boots the dump-capture kernel into single-user
- mode without networking. If you want networking, use "3".
-
-* We generally don' have to bring up a SMP kernel just to capture the
- dump. Hence generally it is useful either to build a UP dump-capture
- kernel or specify maxcpus=1 option while loading dump-capture kernel.
-
-* For s390x there are two kdump modes: If a ELF header is specified with
- the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
- is done on all other architectures. If no elfcorehdr= kernel parameter is
- specified, the s390x kdump kernel dynamically creates the header. The
- second mode has the advantage that for CPU and memory hotplug, kdump has
- not to be reloaded with kexec_load().
-
-* For s390x systems with many attached devices the "cio_ignore" kernel
- parameter should be used for the kdump kernel in order to prevent allocation
- of kernel memory for devices that are not relevant for kdump. The same
- applies to systems that use SCSI/FCP devices. In that case the
- "allow_lun_scan" zfcp module parameter should be set to zero before
- setting FCP devices online.
-
-Kernel Panic
-============
-
-After successfully loading the dump-capture kernel as previously
-described, the system will reboot into the dump-capture kernel if a
-system crash is triggered. Trigger points are located in panic(),
-die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
-
-The following conditions will execute a crash trigger point:
-
-If a hard lockup is detected and "NMI watchdog" is configured, the system
-will boot into the dump-capture kernel ( die_nmi() ).
-
-If die() is called, and it happens to be a thread with pid 0 or 1, or die()
-is called inside interrupt context or die() is called and panic_on_oops is set,
-the system will boot into the dump-capture kernel.
-
-On powerpc systems when a soft-reset is generated, die() is called by all cpus
-and the system will boot into the dump-capture kernel.
-
-For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
-"echo c > /proc/sysrq-trigger" or write a module to force the panic.
-
-Write Out the Dump File
-=======================
-
-After the dump-capture kernel is booted, write out the dump file with
-the following command:
-
- cp /proc/vmcore <dump-file>
-
-You can also access dumped memory as a /dev/oldmem device for a linear
-and raw view. To create the device, use the following command:
-
- mknod /dev/oldmem c 1 12
-
-Use the dd command with suitable options for count, bs, and skip to
-access specific portions of the dump.
-
-To see the entire memory, use the following command:
-
- dd if=/dev/oldmem of=oldmem.001
-
-
-Analysis
-========
-
-Before analyzing the dump image, you should reboot into a stable kernel.
-
-You can do limited analysis using GDB on the dump file copied out of
-/proc/vmcore. Use the debug vmlinux built with -g and run the following
-command:
-
- gdb vmlinux <dump-file>
-
-Stack trace for the task on processor 0, register display, and memory
-display work fine.
-
-Note: GDB cannot analyze core files generated in ELF64 format for x86.
-On systems with a maximum of 4GB of memory, you can generate
-ELF32-format headers using the --elf32-core-headers kernel option on the
-dump kernel.
-
-You can also use the Crash utility to analyze dump files in Kdump
-format. Crash is available on Dave Anderson's site at the following URL:
-
- http://people.redhat.com/~anderson/
-
-
-To Do
-=====
-
-1) Provide relocatable kernels for all architectures to help in maintaining
- multiple kernels for crash_dump, and the same kernel as the system kernel
- can be used to capture the dump.
-
-
-Contact
-=======
-
-Vivek Goyal (vgoyal@redhat.com)
-Maneesh Soni (maneesh@in.ibm.com)
-