mirror of
https://github.com/FEX-Emu/linux.git
synced 2024-12-12 02:40:53 +00:00
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>
This commit is contained in:
Mahesh Salgaonkar
Benjamin Herrenschmidt
parent
e55d7f737d
commit
8e0aa6d436
270
Documentation/powerpc/firmware-assisted-dump.txt
Normal file
270
Documentation/powerpc/firmware-assisted-dump.txt
Normal file
@ -0,0 +1,270 @@
|
||||
|
||||
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.
|
||||
Loading…
Reference in New Issue
Block a user