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Mahesh Salgaonkar8e0aa6d2012-02-16 01:14:14 +00001
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 'fadump_reserve_mem=' which will
59 override the default calculated size. Use this option
60 if default boot memory size is not sufficient for second
61 kernel to boot successfully.
62
63-- After the low memory (boot memory) area has been saved, the
64 firmware will reset PCI and other hardware state. It will
65 *not* clear the RAM. It will then launch the bootloader, as
66 normal.
67
68-- The freshly booted kernel will notice that there is a new
69 node (ibm,dump-kernel) in the device tree, indicating that
70 there is crash data available from a previous boot. During
71 the early boot OS will reserve rest of the memory above
72 boot memory size effectively booting with restricted memory
73 size. This will make sure that the second kernel will not
74 touch any of the dump memory area.
75
76-- User-space tools will read /proc/vmcore to obtain the contents
77 of memory, which holds the previous crashed kernel dump in ELF
78 format. The userspace tools may copy this info to disk, or
79 network, nas, san, iscsi, etc. as desired.
80
81-- Once the userspace tool is done saving dump, it will echo
82 '1' to /sys/kernel/fadump_release_mem to release the reserved
83 memory back to general use, except the memory required for
84 next firmware-assisted dump registration.
85
86 e.g.
87 # echo 1 > /sys/kernel/fadump_release_mem
88
89Please note that the firmware-assisted dump feature
90is only available on Power6 and above systems with recent
91firmware versions.
92
93Implementation details:
94----------------------
95
96During boot, a check is made to see if firmware supports
97this feature on that particular machine. If it does, then
98we check to see if an active dump is waiting for us. If yes
99then everything but boot memory size of RAM is reserved during
100early boot (See Fig. 2). This area is released once we finish
101collecting the dump from user land scripts (e.g. kdump scripts)
102that are run. If there is dump data, then the
103/sys/kernel/fadump_release_mem file is created, and the reserved
104memory is held.
105
106If there is no waiting dump data, then only the memory required
107to hold CPU state, HPTE region, boot memory dump and elfcore
108header, is reserved at the top of memory (see Fig. 1). This area
109is *not* released: this region will be kept permanently reserved,
110so that it can act as a receptacle for a copy of the boot memory
111content in addition to CPU state and HPTE region, in the case a
112crash does occur.
113
114 o Memory Reservation during first kernel
115
116 Low memory Top of memory
117 0 boot memory size |
118 | | |<--Reserved dump area -->|
119 V V | Permanent Reservation V
120 +-----------+----------/ /----------+---+----+-----------+----+
121 | | |CPU|HPTE| DUMP |ELF |
122 +-----------+----------/ /----------+---+----+-----------+----+
123 | ^
124 | |
125 \ /
126 -------------------------------------------
127 Boot memory content gets transferred to
128 reserved area by firmware at the time of
129 crash
130 Fig. 1
131
132 o Memory Reservation during second kernel after crash
133
134 Low memory Top of memory
135 0 boot memory size |
136 | |<------------- Reserved dump area ----------- -->|
137 V V V
138 +-----------+----------/ /----------+---+----+-----------+----+
139 | | |CPU|HPTE| DUMP |ELF |
140 +-----------+----------/ /----------+---+----+-----------+----+
141 | |
142 V V
143 Used by second /proc/vmcore
144 kernel to boot
145 Fig. 2
146
147Currently the dump will be copied from /proc/vmcore to a
148a new file upon user intervention. The dump data available through
149/proc/vmcore will be in ELF format. Hence the existing kdump
150infrastructure (kdump scripts) to save the dump works fine with
151minor modifications.
152
153The tools to examine the dump will be same as the ones
154used for kdump.
155
156How to enable firmware-assisted dump (fadump):
157-------------------------------------
158
1591. Set config option CONFIG_FA_DUMP=y and build kernel.
1602. Boot into linux kernel with 'fadump=on' kernel cmdline option.
1613. Optionally, user can also set 'fadump_reserve_mem=' kernel cmdline
162 to specify size of the memory to reserve for boot memory dump
163 preservation.
164
165NOTE: If firmware-assisted dump fails to reserve memory then it will
166 fallback to existing kdump mechanism if 'crashkernel=' option
167 is set at kernel cmdline.
168
169Sysfs/debugfs files:
170------------
171
172Firmware-assisted dump feature uses sysfs file system to hold
173the control files and debugfs file to display memory reserved region.
174
175Here is the list of files under kernel sysfs:
176
177 /sys/kernel/fadump_enabled
178
179 This is used to display the fadump status.
180 0 = fadump is disabled
181 1 = fadump is enabled
182
183 This interface can be used by kdump init scripts to identify if
184 fadump is enabled in the kernel and act accordingly.
185
186 /sys/kernel/fadump_registered
187
188 This is used to display the fadump registration status as well
189 as to control (start/stop) the fadump registration.
190 0 = fadump is not registered.
191 1 = fadump is registered and ready to handle system crash.
192
193 To register fadump echo 1 > /sys/kernel/fadump_registered and
194 echo 0 > /sys/kernel/fadump_registered for un-register and stop the
195 fadump. Once the fadump is un-registered, the system crash will not
196 be handled and vmcore will not be captured. This interface can be
197 easily integrated with kdump service start/stop.
198
199 /sys/kernel/fadump_release_mem
200
201 This file is available only when fadump is active during
202 second kernel. This is used to release the reserved memory
203 region that are held for saving crash dump. To release the
204 reserved memory echo 1 to it:
205
206 echo 1 > /sys/kernel/fadump_release_mem
207
208 After echo 1, the content of the /sys/kernel/debug/powerpc/fadump_region
209 file will change to reflect the new memory reservations.
210
211 The existing userspace tools (kdump infrastructure) can be easily
212 enhanced to use this interface to release the memory reserved for
213 dump and continue without 2nd reboot.
214
215Here is the list of files under powerpc debugfs:
216(Assuming debugfs is mounted on /sys/kernel/debug directory.)
217
218 /sys/kernel/debug/powerpc/fadump_region
219
220 This file shows the reserved memory regions if fadump is
221 enabled otherwise this file is empty. The output format
222 is:
223 <region>: [<start>-<end>] <reserved-size> bytes, Dumped: <dump-size>
224
225 e.g.
226 Contents when fadump is registered during first kernel
227
228 # cat /sys/kernel/debug/powerpc/fadump_region
229 CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x0
230 HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x0
231 DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x0
232
233 Contents when fadump is active during second kernel
234
235 # cat /sys/kernel/debug/powerpc/fadump_region
236 CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x40020
237 HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x1000
238 DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x10000000
239 : [0x00000010000000-0x0000006ffaffff] 0x5ffb0000 bytes, Dumped: 0x5ffb0000
240
241NOTE: Please refer to Documentation/filesystems/debugfs.txt on
242 how to mount the debugfs filesystem.
243
244
245TODO:
246-----
247 o Need to come up with the better approach to find out more
248 accurate boot memory size that is required for a kernel to
249 boot successfully when booted with restricted memory.
250 o The fadump implementation introduces a fadump crash info structure
251 in the scratch area before the ELF core header. The idea of introducing
252 this structure is to pass some important crash info data to the second
253 kernel which will help second kernel to populate ELF core header with
254 correct data before it gets exported through /proc/vmcore. The current
255 design implementation does not address a possibility of introducing
256 additional fields (in future) to this structure without affecting
257 compatibility. Need to come up with the better approach to address this.
258 The possible approaches are:
259 1. Introduce version field for version tracking, bump up the version
260 whenever a new field is added to the structure in future. The version
261 field can be used to find out what fields are valid for the current
262 version of the structure.
263 2. Reserve the area of predefined size (say PAGE_SIZE) for this
264 structure and have unused area as reserved (initialized to zero)
265 for future field additions.
266 The advantage of approach 1 over 2 is we don't need to reserve extra space.
267---
268Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
269This document is based on the original documentation written for phyp
270assisted dump by Linas Vepstas and Manish Ahuja.