[RFC PATCH 0/4] Flush nvdimm/pmem to memory before machine restart

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

Introduce a new pre_restart notifier chain for callbacks that need to
be executed after the system has been made quiescent with
syscore_shutdown(), before machine restart.

Register pre-restart notifiers to flush pmem areas from CPU data cache
to memory on reboot, immediately before restarting the machine. This
ensures all other CPUs are quiescent before the pmem data is flushed to
memory.

The use-case for this new notifier chain is to preserve tracing data
within pmem areas on systems where the BIOS does not clear memory across
warm reboots.

I did an earlier POC that flushed caches on panic/die oops notifiers [1],
but it did not cover the reboot case. I've been made aware that some
distribution vendors have started shipping their own modified version of
my earlier POC patch. This makes a strong argument for upstreaming this
work.

Link: https://lore.kernel.org/linux-kernel/f6067e3e-a2bc-483d-b214-6e3fe6691279@efficios.com/ [1]
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: nvdimm@lists.linux.dev
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org

Mathieu Desnoyers (4):
  kernel/reboot: Introduce pre_restart notifiers
  nvdimm/pmem: Flush to memory before machine restart
  arm64: Invoke pre_restart notifiers
  x86: Invoke pre_restart notifiers

 arch/arm64/kernel/process.c |  2 ++
 arch/x86/kernel/reboot.c    |  7 +++--
 drivers/nvdimm/pmem.c       | 29 ++++++++++++++++++++-
 drivers/nvdimm/pmem.h       |  2 ++
 include/linux/reboot.h      |  4 +++
 kernel/reboot.c             | 51 +++++++++++++++++++++++++++++++++++++
 6 files changed, 92 insertions(+), 3 deletions(-)

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2.39.2