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authorManish Ahuja <ahuja@austin.ibm.com>2008-03-22 10:33:10 +1100
committerPaul Mackerras <paulus@samba.org>2008-03-26 08:44:06 +1100
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[POWERPC] pseries: phyp dump: Documentation
Basic documentation for hypervisor-assisted dump. Signed-off-by: Linas Vepstas <linasvepstas@gmail.com> Signed-off-by: Manish Ahuja <mahuja@us.ibm.com> Signed-off-by: Paul Mackerras <paulus@samba.org>
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+
+ 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
+as follows:
+
+-- 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
+firmware versions.
+
+Implementation details:
+----------------------
+
+During boot, a check is made to see if firmware supports
+this feature on this particular machine. If it does, then
+we check to see if a active dump is waiting for us. If yes
+then everything but 256 MB of RAM is reserved during early
+boot. This area is released once we collect a dump from user
+land scripts that are run. 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 only the highest
+256MB of the ram is reserved as a scratch area. This area
+is *not* 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
+in /sys/kernel/release_region.
+
+The tools to examine the dump will be same as the ones
+used for kdump.
+
+General notes:
+--------------
+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
+preserve security.
+
+Open issues/ToDo:
+------------
+ 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
+ patches.
+
+ o Still need to write a shell script, to copy the dump away.
+ Currently I am parsing it manually.
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