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+Documentation for kdump - the kexec-based crash dumping solution
+Kdump uses kexec to reboot to a second kernel whenever a dump needs to be taken.
+This second kernel is booted with very little memory. The first kernel reserves
+the section of memory that the second kernel uses. This ensures that on-going
+DMA from the first kernel does not corrupt the second kernel.
+All the necessary information about Core image is encoded in ELF format and
+stored in reserved area of memory before crash. Physical address of start of
+ELF header is passed to new kernel through command line parameter elfcorehdr=.
+On i386, the first 640 KB of physical memory is needed to boot, irrespective
+of where the kernel loads. Hence, this region is backed up by kexec just before
+rebooting into the new kernel.
+In the second kernel, "old memory" can be accessed in two ways.
+- The first one is through a /dev/oldmem device interface. A capture utility
+ can read the device file and write out the memory in raw format. This is raw
+ dump of memory and analysis/capture tool should be intelligent enough to
+ determine where to look for the right information. ELF headers (elfcorehdr=)
+ can become handy here.
+- The second interface is through /proc/vmcore. This exports the dump as an ELF
+ format file which can be written out using any file copy command
+ (cp, scp, etc). Further, gdb can be used to perform limited debugging on
+ the dump file. This method ensures methods ensure that there is correct
+ ordering of the dump pages (corresponding to the first 640 KB that has been
+ relocated).
+1) Download http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz
+ and apply http://lse.sourceforge.net/kdump/patches/kexec-tools-1.101-kdump.patch
+ and after that build the source.
+2) Download and build the appropriate (latest) kexec/kdump (-mm) kernel
+ patchset and apply it to the vanilla kernel tree.
+ Two kernels need to be built in order to get this feature working.
+ A) First kernel:
+ a) Enable "kexec system call" feature (in Processor type and features).
+ b) This kernel's physical load address should be the default value of
+ 0x100000 (0x100000, 1 MB) (in Processor type and features).
+ c) Enable "sysfs file system support" (in Pseudo filesystems).
+ d) Boot into first kernel with the command line parameter "crashkernel=Y@X".
+ Use appropriate values for X and Y. Y denotes how much memory to reserve
+ for the second kernel, and X denotes at what physical address the reserved
+ memory section starts. For example: "crashkernel=64M@16M".
+ B) Second kernel:
+ a) Enable "kernel crash dumps" feature (in Processor type and features).
+ b) Specify a suitable value for "Physical address where the kernel is
+ loaded" (in Processor type and features). Typically this value
+ should be same as X (See option d) above, e.g., 16 MB or 0x1000000.
+ c) Enable "/proc/vmcore support" (Optional, in Pseudo filesystems).
+ Note: Options a) and b) depend upon "Configure standard kernel features
+ (for small systems)" (under General setup).
+ Option a) also depends on CONFIG_HIGHMEM (under Processor
+ type and features).
+ Both option a) and b) are under "Processor type and features".
+3) Boot into the first kernel. You are now ready to try out kexec-based crash
+ dumps.
+4) Load the second kernel to be booted using:
+ kexec -p <second-kernel> --crash-dump --args-linux --append="root=<root-dev>
+ maxcpus=1 init 1"
+ Note: i) <second-kernel> has to be a vmlinux image. bzImage will not work,
+ as of now.
+ ii) By default ELF headers are stored in ELF32 format (for i386). This
+ is sufficient to represent the physical memory up to 4GB. To store
+ headers in ELF64 format, specifiy "--elf64-core-headers" on the
+ kexec command line additionally.
+ iii) For now (or until it is fixed), it's best to build the
+ second-kernel without multi-processor support, i.e., make it
+ a uniprocessor kernel.
+5) System reboots into the second kernel when a panic occurs. A module can be
+ written to force the panic, for testing purposes.
+6) Write out the dump file using
+ cp /proc/vmcore <dump-file>
+ Dump memory can also be accessed as a /dev/oldmem device for a linear/raw
+ view. To create the device, type:
+ mknod /dev/oldmem c 1 12
+ Use "dd" with suitable options for count, bs and skip to access specific
+ portions of the dump.
+ Entire memory: dd if=/dev/oldmem of=oldmem.001
+Limited analysis can be done using gdb on the dump file copied out of
+/proc/vmcore. Use vmlinux built with -g and run
+ gdb vmlinux <dump-file>
+Stack trace for the task on processor 0, register display, memory display
+work fine.
+Note: gdb cannot analyse core files generated in ELF64 format for i386.
+1) Provide a kernel pages filtering mechanism so that core file size is not
+ insane on systems having huge memory banks.
+2) Modify "crash" tool to make it recognize this dump.
+Hariprasad Nellitheertha - hari at in dot ibm dot com
+Vivek Goyal (vgoyal@in.ibm.com)

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