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Subject[RFC PATCH 01/10] fadump: Add documentation for firmware-assisted dump.
From
Date
From: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>

Documentation for firmware-assisted dump. This document is based on the
original documentation written for phyp assisted dump by Linas Vepstas
and Manish Ahuja, with few changes to reflect the current implementation.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
---
Documentation/powerpc/firmware-assisted-dump.txt | 231 ++++++++++++++++++++++
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+
+ Firmware-Assisted Dump
+ ------------------------
+ July 2011
+
+The goal of firmware-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, firmware-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.
+-- Once the dump is copied out, the memory that held the dump
+ is immediately available to the running kernel. A further
+ reboot isn't required.
+
+The above can only be accomplished by coordination with,
+and assistance from the Power firmware. The procedure is
+as follows:
+
+-- The first kernel registers the sections of memory with the
+ Power firmware for dump preservation during OS initialization.
+ This registered sections of memory is reserved by the first
+ kernel during early boot.
+
+-- When a system crashes, the Power firmware will save
+ the low memory (boot memory of size larger of 5% of system RAM
+ or 256MB) of RAM to a previously registered save region. It
+ will also save system registers, and hardware PTE's.
+
+ NOTE: The term 'boot memory' means size of the low memory chunk
+ that is required for a kernel to boot successfully when
+ booted with restricted memory.
+
+-- After the low memory (boot memory) area has been saved, the
+ firmware will reset PCI and other hardware state. It will
+ *not* clear the 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. During
+ the early boot OS will reserve rest of the memory above
+ boot memory size effectively booting with restricted memory
+ size. This will make sure that the second kernel will not
+ touch any of the dump memory area.
+
+-- Userspace tools will read /proc/vmcore to obtain the contents
+ of memory, which holds the previous crashed kernel dump in ELF
+ format. The userspace tools may copy this info to disk, or
+ network, nas, san, iscsi, etc. as desired.
+
+-- Once the userspace tool is done saving dump, it will echo
+ '1' to /sys/kernel/fadump_release_mem to release the reserved
+ memory back to general use, except the memory required for
+ next firmware-assisted dump registration.
+
+ e.g.
+ # echo 1 > /sys/kernel/fadump_release_mem
+
+Please note that the firmware-assisted dump feature
+is only available on Power6 and above systems with recent
+firmware versions.
+
+Implementation details:
+----------------------
+
+During boot, a check is made to see if firmware supports
+this feature on that particular machine. If it does, then
+we check to see if an active dump is waiting for us. If yes
+then everything but boot memory size of RAM is reserved during
+early boot (See Fig. 2). This area is released once we collect a
+dump from user land scripts (kdump scripts) that are run. If
+there is dump data, then the /sys/kernel/fadump_release_mem
+file is created, and the reserved memory is held.
+
+If there is no waiting dump data, then only the memory required
+to hold CPU state, HPTE region, boot memory dump and elfcore
+header, is reserved at the top of memory (see Fig. 1). 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 boot memory
+content in addition to CPU state and HPTE region, in the case a
+crash does occur.
+
+ o Memory Reservation during first kernel
+
+ Low memory Top of memory
+ 0 boot memory size |
+ | | |<--Reserved dump area -->|
+ V V | Permanent Reservation V
+ +-----------+----------/ /----------+---+----+-----------+----+
+ | | |CPU|HPTE| DUMP |ELF |
+ +-----------+----------/ /----------+---+----+-----------+----+
+ | ^
+ | |
+ \ /
+ -------------------------------------------
+ Boot memory content gets transferred to
+ reserved area by firmware at the time of
+ crash
+ Fig. 1
+
+ o Memory Reservation during second kernel after crash
+
+ Low memory Top of memory
+ 0 boot memory size |
+ | |<------------- Reserved dump area ----------- -->|
+ V V V
+ +-----------+----------/ /----------+---+----+-----------+----+
+ | | |CPU|HPTE| DUMP |ELF |
+ +-----------+----------/ /----------+---+----+-----------+----+
+ | |
+ V V
+ Used by second /proc/vmcore
+ kernel to boot
+ Fig. 2
+
+Currently the dump will be copied from /proc/vmcore to a
+a new file upon user intervention. The dump data available through
+/proc/vmcore will be in ELF format. Hence the existing kdump
+infrastructure (kdump scripts) to save the dump works fine
+with minor modifications. The kdump script requires following
+modifications:
+-- During service kdump start if /proc/vmcore entry is not present,
+ look for the existence of /sys/kernel/fadump_enabled and read
+ value exported by it. If value is set to '1' then print
+ success otherwise fallback to existing kexec based kdump.
+
+-- During service kdump start if /proc/vmcore entry is present,
+ execute the existing routine to save the dump. Once the dump
+ is saved, echo 1 > /sys/kernel/fadump_release_mem (if the
+ file exists) to release the reserved memory for general use
+ and continue without rebooting. At this point the memory
+ reservation map will look like as shown in Fig. 1. If the file
+ /sys/kernel/fadump_release_mem is not present then follow
+ the existing routine to reboot into new kernel.
+
+The tools to examine the dump will be same as the ones
+used for kdump.
+
+How to enable firmware-assisted dump (fadump):
+-------------------------------------
+
+1. Set config option CONFIG_FA_DUMP=y and build kernel.
+2. Boot into linux kernel with 'fadump=1' kernel cmdline option.
+
+NOTE: If firmware-assisted dump fails to reserve memory then it will
+ fallback to existing kdump mechanism if 'crashkernel=' option
+ is set at kernel cmdline.
+
+Sysfs files:
+------------
+
+Firmware-assisted dump feature uses sysfs file system to hold
+the control files as well as the files to display memory reserved
+region.
+
+Here is the list of files under kernel sysfs:
+
+ /sys/kernel/fadump_enabled
+
+ This is used to display the fadump status.
+ 0 = fadump is disabled
+ 1 = fadump is enabled
+
+ /sys/kernel/fadump_region
+
+ This file shows the reserved memory regions if fadump is
+ enabled otherwise this file is empty. The output format
+ is:
+ <region>: [<start>-<end>] <reserved-size> bytes, Dumped: <dump-size>
+
+ e.g.
+ Contents when fadump is registered during first kernel
+
+ # cat /sys/kernel/fadump_region
+ CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x0
+ HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x0
+ DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x0
+
+ Contents when fadump is active during second kernel
+
+ # cat /sys/kernel/fadump_region
+ CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x40020
+ HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x1000
+ DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x10000000
+ : [0x00000010000000-0x0000006ffaffff] 0x5ffb0000 bytes, Dumped: 0x5ffb0000
+
+ /sys/kernel/fadump_release_mem
+
+ This file is available only when fadump is active during
+ second kernel. This is used to release the reserved memory
+ region that are held for saving crash dump. To release the
+ reserved memory echo 1 to it:
+
+ echo 1 > /sys/kernel/fadump_release_mem
+
+ After echo 1, the content of the /sys/kernel/fadump_region
+ file will change to reflect the new memory reservations.
+
+TODO:
+-----
+ o Need to come up with the better approach to find out more
+ accurate boot memory size that is required for a kernel to
+ boot successfully when booted with restricted memory.
+ o The fadump implementation introduces a fadump crash info structure
+ in the scratch area before the ELF core header. The idea of introducing
+ this structure is to pass some important crash info data to the second
+ kernel which will help second kernel to populate ELF core header with
+ correct data before it gets exported through /proc/vmcore. The current
+ design implementation does not address a possibility of introducing
+ additional fields (in future) to this structure without affecting
+ compatibility. Need to come up with the better approach to address this.
+ The possible approaches are:
+ 1. Introduce version field for version tracking, bump up the version
+ whenever a new field is added to the structure in future. The version
+ field can be used to find out what fields are valid for the current
+ version of the structure.
+ 2. Reserve the area of predefined size (say PAGE_SIZE) for this
+ structure and have unused area as reserved (initialized to zero)
+ for future field additions.
+ The advantage of approach 1 over 2 is we don't need to reserve extra space.
+---
+Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
+This document is based on the original documentation written for phyp
+assisted dump by Linas Vepstas and Manish Ahuja.


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