fadump: Add documentation for firmware-assisted dump.

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>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Documentation/powerpc/firmware-assisted-dump.txt

+
+                   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.
+
+- Firmware assisted dump (fadump) infrastructure is intended to replace
+  the existing phyp assisted dump.
+- Fadump uses the same firmware interfaces and memory reservation model
+  as phyp assisted dump.
+- Unlike phyp dump, fadump exports the memory dump through /proc/vmcore
+  in the ELF format in the same way as kdump. This helps us reuse the
+  kdump infrastructure for dump capture and filtering.
+- Unlike phyp dump, userspace tool does not need to refer any sysfs
+  interface while reading /proc/vmcore.
+- Unlike phyp dump, fadump allows user to release all the memory reserved
+  for dump, with a single operation of echo 1 > /sys/kernel/fadump_release_mem.
+- Once enabled through kernel boot parameter, fadump can be
+  started/stopped through /sys/kernel/fadump_registered interface (see
+  sysfs files section below) and can be easily integrated with kdump
+  service start/stop init scripts.
+
+Comparing with 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. And therefore,
+   unlike kdump, fadump doesn't need a 2nd reboot to get back
+   the system to the production configuration.
+
+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.
+   These registered sections of memory are 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 the previous registered 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. By default, the boot memory
+         size will be the larger of 5% of system RAM or 256MB.
+         Alternatively, user can also specify boot memory size
+         through boot parameter 'fadump_reserve_mem=' which will
+         override the default calculated size. Use this option
+         if default boot memory size is not sufficient for second
+         kernel to boot successfully.
+
+-- 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.
+
+-- User-space 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 finish
+collecting the dump from user land scripts (e.g. 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 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=on' kernel cmdline option.
+3. Optionally, user can also set 'fadump_reserve_mem=' kernel cmdline
+   to specify size of the memory to reserve for boot memory dump
+   preservation.
+
+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/debugfs files:
+------------
+
+Firmware-assisted dump feature uses sysfs file system to hold
+the control files and debugfs file 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
+
+    This interface can be used by kdump init scripts to identify if
+    fadump is enabled in the kernel and act accordingly.
+
+ /sys/kernel/fadump_registered
+
+    This is used to display the fadump registration status as well
+    as to control (start/stop) the fadump registration.
+    0 = fadump is not registered.
+    1 = fadump is registered and ready to handle system crash.
+
+    To register fadump echo 1 > /sys/kernel/fadump_registered and
+    echo 0 > /sys/kernel/fadump_registered for un-register and stop the
+    fadump. Once the fadump is un-registered, the system crash will not
+    be handled and vmcore will not be captured. This interface can be
+    easily integrated with kdump service start/stop.
+
+ /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/debug/powerpc/fadump_region
+    file will change to reflect the new memory reservations.
+
+    The existing userspace tools (kdump infrastructure) can be easily
+    enhanced to use this interface to release the memory reserved for
+    dump and continue without 2nd reboot.
+
+Here is the list of files under powerpc debugfs:
+(Assuming debugfs is mounted on /sys/kernel/debug directory.)
+
+ /sys/kernel/debug/powerpc/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/debug/powerpc/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/debug/powerpc/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
+
+NOTE: Please refer to Documentation/filesystems/debugfs.txt on
+      how to mount the debugfs filesystem.
+
+
+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.