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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.