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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	
2	                   Firmware-Assisted Dump
3	                   ------------------------
4	                       July 2011
5	
6	The goal of firmware-assisted dump is to enable the dump of
7	a crashed system, and to do so from a fully-reset system, and
8	to minimize the total elapsed time until the system is back
9	in 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	
27	Comparing with kdump or other strategies, firmware-assisted
28	dump 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	
39	The above can only be accomplished by coordination with,
40	and assistance from the Power firmware. The procedure is
41	as 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	
89	Please note that the firmware-assisted dump feature
90	is only available on Power6 and above systems with recent
91	firmware versions.
92	
93	Implementation details:
94	----------------------
95	
96	During boot, a check is made to see if firmware supports
97	this feature on that particular machine. If it does, then
98	we check to see if an active dump is waiting for us. If yes
99	then everything but boot memory size of RAM is reserved during
100	early boot (See Fig. 2). This area is released once we finish
101	collecting the dump from user land scripts (e.g. kdump scripts)
102	that are run. If there is dump data, then the
103	/sys/kernel/fadump_release_mem file is created, and the reserved
104	memory is held.
105	
106	If there is no waiting dump data, then only the memory required
107	to hold CPU state, HPTE region, boot memory dump and elfcore
108	header, is reserved at the top of memory (see Fig. 1). This area
109	is *not* released: this region will be kept permanently reserved,
110	so that it can act as a receptacle for a copy of the boot memory
111	content in addition to CPU state and HPTE region, in the case a
112	crash 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	
147	Currently the dump will be copied from /proc/vmcore to a
148	a new file upon user intervention. The dump data available through
149	/proc/vmcore will be in ELF format. Hence the existing kdump
150	infrastructure (kdump scripts) to save the dump works fine with
151	minor modifications.
152	
153	The tools to examine the dump will be same as the ones
154	used for kdump.
155	
156	How to enable firmware-assisted dump (fadump):
157	-------------------------------------
158	
159	1. Set config option CONFIG_FA_DUMP=y and build kernel.
160	2. Boot into linux kernel with 'fadump=on' kernel cmdline option.
161	3. 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	
165	NOTE: 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	
169	Sysfs/debugfs files:
170	------------
171	
172	Firmware-assisted dump feature uses sysfs file system to hold
173	the control files and debugfs file to display memory reserved region.
174	
175	Here 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	
215	Here 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	
241	NOTE: Please refer to Documentation/filesystems/debugfs.txt on
242	      how to mount the debugfs filesystem.
243	
244	
245	TODO:
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	---
268	Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
269	This document is based on the original documentation written for phyp
270	assisted dump by Linas Vepstas and Manish Ahuja.
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