[RFC PATCH v5 1/9] fadump: Add documentation for firmware-assisted dump.

Mahesh J Salgaonkar mahesh at linux.vnet.ibm.com
Tue Nov 15 10:13:34 EST 2011

From: Mahesh Salgaonkar <mahesh at 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.

Change in v3:
- Modified the documentation to reflect introdunction of fadump_registered
  sysfs file and few minor changes.

Change in v2:
- Modified the documentation to reflect the change of fadump_region
  file under debugfs filesystem.

Signed-off-by: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
 Documentation/powerpc/firmware-assisted-dump.txt |  243 ++++++++++++++++++++++
 1 files changed, 243 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/powerpc/firmware-assisted-dump.txt

diff --git a/Documentation/powerpc/firmware-assisted-dump.txt b/Documentation/powerpc/firmware-assisted-dump.txt
new file mode 100644
index 0000000..3248b5d
--- /dev/null
+++ b/Documentation/powerpc/firmware-assisted-dump.txt
@@ -0,0 +1,243 @@
+                   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.
+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. 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.
+   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.
+-- 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=1' 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
+ /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.
+ /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.
+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.
+ 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 at 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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