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Documentation/powerpc: update fadump implementation details
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The patch titled ("powerpc: make fadump resilient with memory add/remove
events") has made significant changes to the implementation of fadump,
particularly on elfcorehdr creation and fadump crash info header
structure. Therefore, updating the fadump implementation documentation
to reflect those changes.

Following updates are done to firmware assisted dump documentation:

1. The elfcorehdr is no longer stored after fadump HDR in the reserved
   dump area. Instead, the second kernel dynamically allocates memory
   for the elfcorehdr within the address range from 0 to the boot memory
   size. Therefore, update figures 1 and 2 of Memory Reservation during
   the first and second kernels to reflect this change.

2. A version field has been added to the fadump header to maintain
   backward compatibility without changing the fadump header magic
   number in the future. Therefore, remove the corresponding TODO from
   the document.

To: [email protected]
Signed-off-by: Sourabh Jain <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]
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@sourabhjains @mpe
sourabhjains authored and mpe committed Oct 22, 2023
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Showing 1 changed file with 42 additions and 49 deletions.
91 changes: 42 additions & 49 deletions Documentation/powerpc/firmware-assisted-dump.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -134,12 +134,12 @@ that are run. If there is dump data, then the
memory is held.

If there is no waiting dump data, then only the memory required to
hold CPU state, HPTE region, boot memory dump, FADump header and
elfcore header, is usually reserved at an offset greater than boot
memory size (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.
hold CPU state, HPTE region, boot memory dump, and FADump header is
usually reserved at an offset greater than boot memory size (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.

Since this reserved memory area is used only after the system crash,
there is no point in blocking this significant chunk of memory from
Expand All @@ -153,22 +153,22 @@ that were present in CMA region::

o Memory Reservation during first kernel

Low memory Top of memory
0 boot memory size |<--- Reserved dump area --->| |
| | | Permanent Reservation | |
V V | | V
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
| | |///|////| DUMP | HDR | ELF |////| |
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
| ^ ^ ^ ^ ^
| | | | | |
\ CPU HPTE / | |
------------------------------ | |
Boot memory content gets transferred | |
to reserved area by firmware at the | |
time of crash. | |
FADump Header |
(meta area) |
Low memory Top of memory
0 boot memory size |<------ Reserved dump area ----->| |
| | | Permanent Reservation | |
V V | | V
+-----------+-----/ /---+---+----+-----------+-------+----+-----+
| | |///|////| DUMP | HDR |////| |
+-----------+-----/ /---+---+----+-----------+-------+----+-----+
| ^ ^ ^ ^ ^
| | | | | |
\ CPU HPTE / | |
-------------------------------- | |
Boot memory content gets transferred | |
to reserved area by firmware at the | |
time of crash. | |
FADump Header |
(meta area) |
|
|
Metadata: This area holds a metadata structure whose
Expand All @@ -186,20 +186,33 @@ that were present in CMA region::
0 boot memory size |
| |<------------ Crash preserved area ------------>|
V V |<--- Reserved dump area --->| |
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
| | |///|////| DUMP | HDR | ELF |////| |
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
| |
V V
Used by second /proc/vmcore
kernel to boot
+----+---+--+-----/ /---+---+----+-------+-----+-----+-------+
| |ELF| | |///|////| DUMP | HDR |/////| |
+----+---+--+-----/ /---+---+----+-------+-----+-----+-------+
| | | | | |
----- ------------------------------ ---------------
\ | |
\ | |
\ | |
\ | ----------------------------
\ | /
\ | /
\ | /
/proc/vmcore


+---+
|///| -> Regions (CPU, HPTE & Metadata) marked like this in the above
+---+ figures are not always present. For example, OPAL platform
does not have CPU & HPTE regions while Metadata region is
not supported on pSeries currently.

+---+
|ELF| -> elfcorehdr, it is created in second kernel after crash.
+---+

Note: Memory from 0 to the boot memory size is used by second kernel

Fig. 2


Expand Down Expand Up @@ -353,26 +366,6 @@ TODO:
- 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.
- 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 <[email protected]>

Expand Down

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