[PATCH 1/8] pseries: phyp dump: Docmentation
ahuja at austin.ibm.com
Wed Jan 23 06:26:58 EST 2008
Basic documentation for hypervisor-assisted dump.
Signed-off-by: Linas Vepstas <linasvepstas at gmail.com>
Signed-off-by: Manish Ahuja <mahuja at us.ibm.com>
Documentation/powerpc/phyp-assisted-dump.txt | 129 +++++++++++++++++++++++++++
1 file changed, 129 insertions(+)
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ 2.6.24-rc5/Documentation/powerpc/phyp-assisted-dump.txt 2008-01-07 18:05:46.000000000 -0600
@@ -0,0 +1,129 @@
+ Hypervisor-Assisted Dump
+ November 2007
+The goal of hypervisor-assisted dump is to enable the dump of
+a crashed system, and to do so from a fully-reset system, and
+to minimize the total elapsed time until the system is back
+in production use.
+As compared to kdump or other strategies, hypervisor-assisted
+dump offers several strong, practical advantages:
+-- Unlike kdump, the system has been reset, and loaded
+ with a fresh copy of the kernel. In particular,
+ PCI and I/O devices have been reinitialized and are
+ in a clean, consistent state.
+-- As the dump is performed, the dumped memory becomes
+ immediately available to the system for normal use.
+-- After the dump is completed, no further reboots are
+ required; the system will be fully usable, and running
+ in it's normal, production mode on it normal kernel.
+The above can only be accomplished by coordination with,
+and assistance from the hypervisor. The procedure is
+-- When a system crashes, the hypervisor will save
+ the low 256MB of RAM to a previously registered
+ save region. It will also save system state, system
+ registers, and hardware PTE's.
+-- After the low 256MB area has been saved, the
+ hypervisor will reset PCI and other hardware state.
+ It will *not* clear RAM. It will then launch the
+ bootloader, as normal.
+-- The freshly booted kernel will notice that there
+ is a new node (ibm,dump-kernel) in the device tree,
+ indicating that there is crash data available from
+ a previous boot. It will boot into only 256MB of RAM,
+ reserving the rest of system memory.
+-- Userspace tools will parse /sys/kernel/release_region
+ and read /proc/vmcore to obtain the contents of memory,
+ which holds the previous crashed kernel. The userspace
+ tools may copy this info to disk, or network, nas, san,
+ iscsi, etc. as desired.
+ For Example: the values in /sys/kernel/release-region
+ would look something like this (address-range pairs).
+ CPU:0x177fee000-0x10000: HPTE:0x177ffe020-0x1000: /
+ DUMP:0x177fff020-0x10000000, 0x10000000-0x16F1D370A
+-- As the userspace tools complete saving a portion of
+ dump, they echo an offset and size to
+ /sys/kernel/release_region to release the reserved
+ memory back to general use.
+ An example of this is:
+ "echo 0x40000000 0x10000000 > /sys/kernel/release_region"
+ which will release 256MB at the 1GB boundary.
+Please note that the hypervisor-assisted dump feature
+is only available on Power6-based systems with recent
+In order for this scheme to work, memory needs to be reserved
+quite early in the boot cycle. However, access to the device
+tree this early in the boot cycle is difficult, and device-tree
+access is needed to determine if there is a crash data waiting.
+To work around this problem, all but 256MB of RAM is reserved
+during early boot. A short while later in boot, a check is made
+to determine if there is dump data waiting. If there isn't,
+then the reserved memory is released to general kernel use.
+If there is dump data, then the /sys/kernel/release_region
+file is created, and the reserved memory is held.
+If there is no waiting dump data, then all but 256MB of the
+reserved ram will be released for general kernel use. The
+highest 256 MB of RAM will *not* be released: this region
+will be kept permanently reserved, so that it can act as
+a receptacle for a copy of the low 256MB in the case a crash
+does occur. See, however, "open issues" below, as to whether
+such a reserved region is really needed.
+Currently the dump will be copied from /proc/vmcore to a
+a new file upon user intervention. The starting address
+to be read and the range for each data point in provided
+The tools to examine the dump will be same as the ones
+used for kdump.
+Security: please note that there are potential security issues
+with any sort of dump mechanism. In particular, plaintext
+(unencrypted) data, and possibly passwords, may be present in
+the dump data. Userspace tools must take adequate precautions to
+ o The various code paths that tell the hypervisor that a crash
+ occurred, vs. it simply being a normal reboot, should be
+ reviewed, and possibly clarified/fixed.
+ o Instead of using /sys/kernel, should there be a /sys/dump
+ instead? There is a dump_subsys being created by the s390 code,
+ perhaps the pseries code should use a similar layout as well.
+ o Is reserving a 256MB region really required? The goal of
+ reserving a 256MB scratch area is to make sure that no
+ important crash data is clobbered when the hypervisor
+ save low mem to the scratch area. But, if one could assure
+ that nothing important is located in some 256MB area, then
+ it would not need to be reserved. Something that can be
+ improved in subsequent versions.
+ o Still working the kdump team to integrate this with kdump,
+ some work remains but this would not affect the current
+ o Still need to write a shell script, to copy the dump away.
+ Currently I am parsing it manually.
More information about the Linuxppc-dev