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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 its normal, production mode on its 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.