128 lines
4.9 KiB
Plaintext
128 lines
4.9 KiB
Plaintext
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Hypervisor-Assisted Dump
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------------------------
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November 2007
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The goal of hypervisor-assisted dump is to enable the dump of
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a crashed system, and to do so from a fully-reset system, and
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to minimize the total elapsed time until the system is back
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in production use.
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As compared to kdump or other strategies, hypervisor-assisted
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dump offers several strong, practical advantages:
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-- Unlike kdump, the system has been reset, and loaded
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with a fresh copy of the kernel. In particular,
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PCI and I/O devices have been reinitialized and are
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in a clean, consistent state.
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-- As the dump is performed, the dumped memory becomes
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immediately available to the system for normal use.
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-- After the dump is completed, no further reboots are
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required; the system will be fully usable, and running
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in it's normal, production mode on it normal kernel.
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The above can only be accomplished by coordination with,
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and assistance from the hypervisor. The procedure is
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as follows:
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-- When a system crashes, the hypervisor will save
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the low 256MB of RAM to a previously registered
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save region. It will also save system state, system
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registers, and hardware PTE's.
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-- After the low 256MB area has been saved, the
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hypervisor will reset PCI and other hardware state.
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It will *not* clear RAM. It will then launch the
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bootloader, as normal.
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-- The freshly booted kernel will notice that there
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is a new node (ibm,dump-kernel) in the device tree,
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indicating that there is crash data available from
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a previous boot. It will boot into only 256MB of RAM,
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reserving the rest of system memory.
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-- Userspace tools will parse /sys/kernel/release_region
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and read /proc/vmcore to obtain the contents of memory,
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which holds the previous crashed kernel. The userspace
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tools may copy this info to disk, or network, nas, san,
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iscsi, etc. as desired.
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For Example: the values in /sys/kernel/release-region
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would look something like this (address-range pairs).
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CPU:0x177fee000-0x10000: HPTE:0x177ffe020-0x1000: /
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DUMP:0x177fff020-0x10000000, 0x10000000-0x16F1D370A
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-- As the userspace tools complete saving a portion of
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dump, they echo an offset and size to
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/sys/kernel/release_region to release the reserved
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memory back to general use.
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An example of this is:
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"echo 0x40000000 0x10000000 > /sys/kernel/release_region"
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which will release 256MB at the 1GB boundary.
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Please note that the hypervisor-assisted dump feature
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is only available on Power6-based systems with recent
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firmware versions.
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Implementation details:
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----------------------
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During boot, a check is made to see if firmware supports
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this feature on this particular machine. If it does, then
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we check to see if a active dump is waiting for us. If yes
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then everything but 256 MB of RAM is reserved during early
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boot. This area is released once we collect a dump from user
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land scripts that are run. If there is dump data, then
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the /sys/kernel/release_region file is created, and
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the reserved memory is held.
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If there is no waiting dump data, then only the highest
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256MB of the ram is reserved as a scratch area. This area
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is *not* released: this region will be kept permanently
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reserved, so that it can act as a receptacle for a copy
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of the low 256MB in the case a crash does occur. See,
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however, "open issues" below, as to whether
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such a reserved region is really needed.
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Currently the dump will be copied from /proc/vmcore to a
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a new file upon user intervention. The starting address
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to be read and the range for each data point in provided
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in /sys/kernel/release_region.
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The tools to examine the dump will be same as the ones
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used for kdump.
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General notes:
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--------------
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Security: please note that there are potential security issues
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with any sort of dump mechanism. In particular, plaintext
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(unencrypted) data, and possibly passwords, may be present in
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the dump data. Userspace tools must take adequate precautions to
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preserve security.
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Open issues/ToDo:
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------------
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o The various code paths that tell the hypervisor that a crash
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occurred, vs. it simply being a normal reboot, should be
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reviewed, and possibly clarified/fixed.
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o Instead of using /sys/kernel, should there be a /sys/dump
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instead? There is a dump_subsys being created by the s390 code,
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perhaps the pseries code should use a similar layout as well.
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o Is reserving a 256MB region really required? The goal of
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reserving a 256MB scratch area is to make sure that no
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important crash data is clobbered when the hypervisor
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save low mem to the scratch area. But, if one could assure
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that nothing important is located in some 256MB area, then
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it would not need to be reserved. Something that can be
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improved in subsequent versions.
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o Still working the kdump team to integrate this with kdump,
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some work remains but this would not affect the current
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patches.
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o Still need to write a shell script, to copy the dump away.
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Currently I am parsing it manually.
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