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.