[PATCH] kdump: Documentation for Kdump

This patch contains the documentation for the kexec based crash dump tool.

Quick kdump-howto
================================================================

1) Download and build kexec-tools.

2) Download and build the latest kexec/kdump (-mm) kernel patchset.
   Two kernels need to be built in order to get this feature working.

  A) First kernel:
   a) Enable "kexec system call" feature:
	CONFIG_KEXEC=y
   b) Physical load address (use default):
	CONFIG_PHYSICAL_START=0x100000
   c) Enable "sysfs file system support":
	CONFIG_SYSFS=y
   d) Boot into first kernel with the command line parameter "crashkernel=Y@X":
      For example: "crashkernel=64M@16M".

  B) Second kernel:
   a) Enable "kernel crash dumps" feature:
	CONFIG_CRASH_DUMP=y
   b) Physical load addreess, use same load address as X in "crashkernel"
      kernel parameter in d) above, e.g., 16 MB or 0x1000000.
	CONFIG_PHYSICAL_START=0x1000000
   c) Enable "/proc/vmcore support" (Optional, in Pseudo filesystems).
	CONFIG_PROC_VMCORE=y

3) Boot into the first kernel.

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"

5) System reboots into the second kernel when a panic occurs. A module can be
   written to force the panic, for testing purposes.

6) See Documentation/kdump.txt for how to read the first kernel's
   memory image and how to analyze it.

Signed-off-by: Hariprasad Nellitheertha <hari@in.ibm.com>
Signed-off-by: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
Signed-off-by: randy_dunlap <rdunlap@xenotime.net>
Signed-off-by: Maneesh Soni <maneesh@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 135 insertions, 0 deletions

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
new file mode 100644
index 00000000000..b0f412e4279
--- /dev/null
+++ b/Documentation/kdump/kdump.txt
@@ -0,0 +1,135 @@
+Documentation for kdump - the kexec-based crash dumping solution
+================================================================
+
+DESIGN
+======
+
+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).
+
+SETUP
+=====
+
+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).
+	CONFIG_KEXEC=y
+   b) This kernel's physical load address should be the default value of
+      0x100000 (0x100000, 1 MB) (in Processor type and features).
+	CONFIG_PHYSICAL_START=0x100000
+   c) Enable "sysfs file system support" (in Pseudo filesystems).
+	CONFIG_SYSFS=y
+   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).
+	CONFIG_CRASH_DUMP=y
+   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.
+	CONFIG_PHYSICAL_START=0x1000000
+   c) Enable "/proc/vmcore support" (Optional, in Pseudo filesystems).
+	CONFIG_PROC_VMCORE=y
+
+  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
+
+ANALYSIS
+========
+
+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.
+
+TODO
+====
+
+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.
+
+CONTACT
+=======
+
+Hariprasad Nellitheertha - hari at in dot ibm dot com
+Vivek Goyal (vgoyal@in.ibm.com)