[RFC/PATCH 7/17][Kdump] Documentation for Kdump

[Vivek Goyal <vgoyal@in.ibm.com>]

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• Subject: No Subject
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• Date: Mon, 28 Mar 2005 17:37:11 +0530

From: "Vivek Goyal" <vgoyal@in.ibm.com>

o Updated the documentation.

From: "Eric W. Biederman" <ebiederm@xmission.com>

I have addressed the worst of the documentation changes that come about from
the current refacatoring.

From: Hariprasad Nellitheertha <hari@in.ibm.com>

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

 linux-2.6.12-rc1-mm1-16M-root/Documentation/00-INDEX  |    2 
 linux-2.6.12-rc1-mm1-16M-root/Documentation/kdump.txt |  118 ++++++++++++++++++
 2 files changed, 120 insertions(+)

diff -puN Documentation/00-INDEX~crashdump-documentation Documentation/00-INDEX
--- linux-2.6.12-rc1-mm1-16M/Documentation/00-INDEX~crashdump-documentation	2005-03-22 16:24:17.000000000 +0530
+++ linux-2.6.12-rc1-mm1-16M-root/Documentation/00-INDEX	2005-03-22 16:24:16.000000000 +0530
@@ -140,6 +140,8 @@ java.txt
 	- info on the in-kernel binary support for Java(tm).
 kbuild/
 	- directory with info about the kernel build process.
+kdumpt.txt
+       - mini HowTo on getting the crash dump code to work.
 kernel-doc-nano-HOWTO.txt
 	- mini HowTo on generation and location of kernel documentation files.
 kernel-docs.txt
diff -puN /dev/null Documentation/kdump.txt
--- /dev/null	2004-02-24 02:32:56.000000000 +0530
+++ linux-2.6.12-rc1-mm1-16M-root/Documentation/kdump.txt	2005-03-22 18:19:43.000000000 +0530
@@ -0,0 +1,118 @@
+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, first 640k of physical memory is needed to boot, irrespctive of where
+the kernel loads at. 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 640k that has been
+  relocated).
+
+SETUP
+=====
+
+1) Obtain the appropriate -mm tree patch and apply it on to the vanilla
+   kernel tree.
+
+2) Obtain appropriate version of kexec-tools.
+
+3) Two kernels need to be built in order to get this feature working.
+
+   First kernel:
+   a) Enable "kexec system call" feature.
+   b) Enable "sysfs file system support" (Pseudo filesystems).
+   c) Boot into first kernel with command line "crashkernel=Y@X".  Put
+      appropriate values for X and Y. Y denotes, how much memory to reserve for
+      second kernel, and X denotes at what physical address reserved memory
+      section starts. For example, crashkernel=48M@16M.
+
+   Second kernel:
+   a) Enable "kernel crash dumps" feature.
+   b) Specifiy a suitable value for "Physical address where the kernel is
+      loaded". Typically this value should be same as X (See option c) above).
+   c) Enable "/proc/vmcore support" (Optional).
+
+      Note: Option a) and b) depend upon "Configure standard kernel feature
+            (for small systems)".
+	    Option a) also depends on CONFIG_HIGHMEM.
+	    Both option a) and b) are under "Processor Types 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 kexec
+	    command line additionally.
+
+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 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.
+
+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 can not 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)
_

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