powerpc: make fadump resilient with memory add/remove events

Due to changes in memory resources caused by either memory hotplug or
online/offline events, the elfcorehdr, which describes the CPUs and
memory of the crashed kernel to the kernel that collects the dump (known
as second/fadump kernel), becomes outdated. Consequently, attempting
dump collection with an outdated elfcorehdr can lead to failed or
inaccurate dump collection.

Memory hotplug or online/offline events is referred as memory add/remove
events in reset of the commit message.

The current solution to address the aforementioned issue is as follows:
Monitor memory add/remove events in userspace using udev rules, and
re-register fadump whenever there are changes in memory resources. This
leads to the creation of a new elfcorehdr with updated system memory
information.

There are several notable issues associated with re-registering fadump
for every memory add/remove events.

1. Bulk memory add/remove events with udev-based fadump re-registration
   can lead to race conditions and, more importantly, it creates a wide
   window during which fadump is inactive until all memory add/remove
   events are settled.
2. Re-registering fadump for every memory add/remove event is
   inefficient.
3. The memory for elfcorehdr is allocated based on the memblock regions
   available during early boot and remains fixed thereafter. However, if
   elfcorehdr is later recreated with additional memblock regions, its
   size will increase, potentially leading to memory corruption.

Address the aforementioned challenges by shifting the creation of
elfcorehdr from the first kernel (also referred as the crashed kernel),
where it was created and frequently recreated for every memory
add/remove event, to the fadump kernel. As a result, the elfcorehdr only
needs to be created once, thus eliminating the necessity to re-register
fadump during memory add/remove events.

At present, the first kernel prepares fadump header and stores it in the
fadump reserved area. The fadump header includes the start address of
the elfcorehdr, crashing CPU details, and other relevant information. In
the event of a crash in the first kernel, the second/fadump boots and
accesses the fadump header prepared by the first kernel. It then
performs the following steps in a platform-specific function
[rtas|opal]_fadump_process:

1. Sanity check for fadump header
2. Update CPU notes in elfcorehdr

Along with the above, update the setup_fadump()/fadump.c to create
elfcorehdr and set its address to the global variable elfcorehdr_addr
for the vmcore module to process it in the second/fadump kernel.

Section below outlines the information required to create the elfcorehdr
and the changes made to make it available to the fadump kernel if it's
not already.

To create elfcorehdr, the following crashed kernel information is
required: CPU notes, vmcoreinfo, and memory ranges.

At present, the CPU notes are already prepared in the fadump kernel, so
no changes are needed in that regard. The fadump kernel has access to
all crashed kernel memory regions, including boot memory regions that
are relocated by firmware to fadump reserved areas, so no changes for
that either. However, it is necessary to add new members to the fadump
header, i.e., the 'fadump_crash_info_header' structure, in order to pass
the crashed kernel's vmcoreinfo address and its size to fadump kernel.

In addition to the vmcoreinfo address and size, there are a few other
attributes also added to the fadump_crash_info_header structure.

1. version:
   It stores the fadump header version, which is currently set to 1.
   This provides flexibility to update the fadump crash info header in
   the future without changing the magic number. For each change in the
   fadump header, the version will be increased. This will help the
   updated kernel determine how to handle kernel dumps from older
   kernels. The magic number remains relevant for checking fadump header
   corruption.

2. pt_regs_sz/cpu_mask_sz:
   Store size of pt_regs and cpu_mask structure of first kernel. These
   attributes are used to prevent dump processing if the sizes of
   pt_regs or cpu_mask structure differ between the first and fadump
   kernels.

Note: if either first/crashed kernel or second/fadump kernel do not have
the changes introduced here then kernel fail to collect the dump and
prints relevant error message on the console.

Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240422195932.1583833-2-sourabhjain@linux.ibm.com

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