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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://horms.net/projects/kexec/
+
+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)
+