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1
2 Firmware-Assisted Dump
3 ------------------------
4 July 2011
5
6The goal of firmware-assisted dump is to enable the dump of
7a crashed system, and to do so from a fully-reset system, and
8to minimize the total elapsed time until the system is back
9in production use.
10
11- Firmware assisted dump (fadump) infrastructure is intended to replace
12 the existing phyp assisted dump.
13- Fadump uses the same firmware interfaces and memory reservation model
14 as phyp assisted dump.
15- Unlike phyp dump, fadump exports the memory dump through /proc/vmcore
16 in the ELF format in the same way as kdump. This helps us reuse the
17 kdump infrastructure for dump capture and filtering.
18- Unlike phyp dump, userspace tool does not need to refer any sysfs
19 interface while reading /proc/vmcore.
20- Unlike phyp dump, fadump allows user to release all the memory reserved
21 for dump, with a single operation of echo 1 > /sys/kernel/fadump_release_mem.
22- Once enabled through kernel boot parameter, fadump can be
23 started/stopped through /sys/kernel/fadump_registered interface (see
24 sysfs files section below) and can be easily integrated with kdump
25 service start/stop init scripts.
26
27Comparing with kdump or other strategies, firmware-assisted
28dump offers several strong, practical advantages:
29
30-- Unlike kdump, the system has been reset, and loaded
31 with a fresh copy of the kernel. In particular,
32 PCI and I/O devices have been reinitialized and are
33 in a clean, consistent state.
34-- Once the dump is copied out, the memory that held the dump
35 is immediately available to the running kernel. And therefore,
36 unlike kdump, fadump doesn't need a 2nd reboot to get back
37 the system to the production configuration.
38
39The above can only be accomplished by coordination with,
40and assistance from the Power firmware. The procedure is
41as follows:
42
43-- The first kernel registers the sections of memory with the
44 Power firmware for dump preservation during OS initialization.
45 These registered sections of memory are reserved by the first
46 kernel during early boot.
47
48-- When a system crashes, the Power firmware will save
49 the low memory (boot memory of size larger of 5% of system RAM
50 or 256MB) of RAM to the previous registered region. It will
51 also save system registers, and hardware PTE's.
52
53 NOTE: The term 'boot memory' means size of the low memory chunk
54 that is required for a kernel to boot successfully when
55 booted with restricted memory. By default, the boot memory
56 size will be the larger of 5% of system RAM or 256MB.
57 Alternatively, user can also specify boot memory size
58 through boot parameter 'crashkernel=' which will override
59 the default calculated size. Use this option if default
60 boot memory size is not sufficient for second kernel to
61 boot successfully. For syntax of crashkernel= parameter,
62 refer to Documentation/kdump/kdump.txt. If any offset is
63 provided in crashkernel= parameter, it will be ignored
64 as fadump uses a predefined offset to reserve memory
65 for boot memory dump preservation in case of a crash.
66
67-- After the low memory (boot memory) area has been saved, the
68 firmware will reset PCI and other hardware state. It will
69 *not* clear the RAM. It will then launch the bootloader, as
70 normal.
71
72-- The freshly booted kernel will notice that there is a new
73 node (ibm,dump-kernel) in the device tree, indicating that
74 there is crash data available from a previous boot. During
75 the early boot OS will reserve rest of the memory above
76 boot memory size effectively booting with restricted memory
77 size. This will make sure that the second kernel will not
78 touch any of the dump memory area.
79
80-- User-space tools will read /proc/vmcore to obtain the contents
81 of memory, which holds the previous crashed kernel dump in ELF
82 format. The userspace tools may copy this info to disk, or
83 network, nas, san, iscsi, etc. as desired.
84
85-- Once the userspace tool is done saving dump, it will echo
86 '1' to /sys/kernel/fadump_release_mem to release the reserved
87 memory back to general use, except the memory required for
88 next firmware-assisted dump registration.
89
90 e.g.
91 # echo 1 > /sys/kernel/fadump_release_mem
92
93Please note that the firmware-assisted dump feature
94is only available on Power6 and above systems with recent
95firmware versions.
96
97Implementation details:
98----------------------
99
100During boot, a check is made to see if firmware supports
101this feature on that particular machine. If it does, then
102we check to see if an active dump is waiting for us. If yes
103then everything but boot memory size of RAM is reserved during
104early boot (See Fig. 2). This area is released once we finish
105collecting the dump from user land scripts (e.g. kdump scripts)
106that are run. If there is dump data, then the
107/sys/kernel/fadump_release_mem file is created, and the reserved
108memory is held.
109
110If there is no waiting dump data, then only the memory required
111to hold CPU state, HPTE region, boot memory dump and elfcore
112header, is usually reserved at an offset greater than boot memory
113size (see Fig. 1). This area is *not* released: this region will
114be kept permanently reserved, so that it can act as a receptacle
115for a copy of the boot memory content in addition to CPU state
116and HPTE region, in the case a crash does occur.
117
118 o Memory Reservation during first kernel
119
120 Low memory Top of memory
121 0 boot memory size |
122 | | |<--Reserved dump area -->| |
123 V V | Permanent Reservation | V
124 +-----------+----------/ /---+---+----+-----------+----+------+
125 | | |CPU|HPTE| DUMP |ELF | |
126 +-----------+----------/ /---+---+----+-----------+----+------+
127 | ^
128 | |
129 \ /
130 -------------------------------------------
131 Boot memory content gets transferred to
132 reserved area by firmware at the time of
133 crash
134 Fig. 1
135
136 o Memory Reservation during second kernel after crash
137
138 Low memory Top of memory
139 0 boot memory size |
140 | |<------------- Reserved dump area ----------- -->|
141 V V V
142 +-----------+----------/ /---+---+----+-----------+----+------+
143 | | |CPU|HPTE| DUMP |ELF | |
144 +-----------+----------/ /---+---+----+-----------+----+------+
145 | |
146 V V
147 Used by second /proc/vmcore
148 kernel to boot
149 Fig. 2
150
151Currently the dump will be copied from /proc/vmcore to a
152a new file upon user intervention. The dump data available through
153/proc/vmcore will be in ELF format. Hence the existing kdump
154infrastructure (kdump scripts) to save the dump works fine with
155minor modifications.
156
157The tools to examine the dump will be same as the ones
158used for kdump.
159
160How to enable firmware-assisted dump (fadump):
161-------------------------------------
162
1631. Set config option CONFIG_FA_DUMP=y and build kernel.
1642. Boot into linux kernel with 'fadump=on' kernel cmdline option.
1653. Optionally, user can also set 'crashkernel=' kernel cmdline
166 to specify size of the memory to reserve for boot memory dump
167 preservation.
168
169NOTE: 1. 'fadump_reserve_mem=' parameter has been deprecated. Instead
170 use 'crashkernel=' to specify size of the memory to reserve
171 for boot memory dump preservation.
172 2. If firmware-assisted dump fails to reserve memory then it
173 will fallback to existing kdump mechanism if 'crashkernel='
174 option is set at kernel cmdline.
175
176Sysfs/debugfs files:
177------------
178
179Firmware-assisted dump feature uses sysfs file system to hold
180the control files and debugfs file to display memory reserved region.
181
182Here is the list of files under kernel sysfs:
183
184 /sys/kernel/fadump_enabled
185
186 This is used to display the fadump status.
187 0 = fadump is disabled
188 1 = fadump is enabled
189
190 This interface can be used by kdump init scripts to identify if
191 fadump is enabled in the kernel and act accordingly.
192
193 /sys/kernel/fadump_registered
194
195 This is used to display the fadump registration status as well
196 as to control (start/stop) the fadump registration.
197 0 = fadump is not registered.
198 1 = fadump is registered and ready to handle system crash.
199
200 To register fadump echo 1 > /sys/kernel/fadump_registered and
201 echo 0 > /sys/kernel/fadump_registered for un-register and stop the
202 fadump. Once the fadump is un-registered, the system crash will not
203 be handled and vmcore will not be captured. This interface can be
204 easily integrated with kdump service start/stop.
205
206 /sys/kernel/fadump_release_mem
207
208 This file is available only when fadump is active during
209 second kernel. This is used to release the reserved memory
210 region that are held for saving crash dump. To release the
211 reserved memory echo 1 to it:
212
213 echo 1 > /sys/kernel/fadump_release_mem
214
215 After echo 1, the content of the /sys/kernel/debug/powerpc/fadump_region
216 file will change to reflect the new memory reservations.
217
218 The existing userspace tools (kdump infrastructure) can be easily
219 enhanced to use this interface to release the memory reserved for
220 dump and continue without 2nd reboot.
221
222Here is the list of files under powerpc debugfs:
223(Assuming debugfs is mounted on /sys/kernel/debug directory.)
224
225 /sys/kernel/debug/powerpc/fadump_region
226
227 This file shows the reserved memory regions if fadump is
228 enabled otherwise this file is empty. The output format
229 is:
230 <region>: [<start>-<end>] <reserved-size> bytes, Dumped: <dump-size>
231
232 e.g.
233 Contents when fadump is registered during first kernel
234
235 # cat /sys/kernel/debug/powerpc/fadump_region
236 CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x0
237 HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x0
238 DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x0
239
240 Contents when fadump is active during second kernel
241
242 # cat /sys/kernel/debug/powerpc/fadump_region
243 CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x40020
244 HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x1000
245 DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x10000000
246 : [0x00000010000000-0x0000006ffaffff] 0x5ffb0000 bytes, Dumped: 0x5ffb0000
247
248NOTE: Please refer to Documentation/filesystems/debugfs.txt on
249 how to mount the debugfs filesystem.
250
251
252TODO:
253-----
254 o Need to come up with the better approach to find out more
255 accurate boot memory size that is required for a kernel to
256 boot successfully when booted with restricted memory.
257 o The fadump implementation introduces a fadump crash info structure
258 in the scratch area before the ELF core header. The idea of introducing
259 this structure is to pass some important crash info data to the second
260 kernel which will help second kernel to populate ELF core header with
261 correct data before it gets exported through /proc/vmcore. The current
262 design implementation does not address a possibility of introducing
263 additional fields (in future) to this structure without affecting
264 compatibility. Need to come up with the better approach to address this.
265 The possible approaches are:
266 1. Introduce version field for version tracking, bump up the version
267 whenever a new field is added to the structure in future. The version
268 field can be used to find out what fields are valid for the current
269 version of the structure.
270 2. Reserve the area of predefined size (say PAGE_SIZE) for this
271 structure and have unused area as reserved (initialized to zero)
272 for future field additions.
273 The advantage of approach 1 over 2 is we don't need to reserve extra space.
274---
275Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
276This document is based on the original documentation written for phyp
277assisted dump by Linas Vepstas and Manish Ahuja.