[PATCH v10 0/2] Detect stalls on guest vCPUS

[Sebastian Ene <sebastianene@google.com>]

This is a re-work from v9 and addresses the comments received from Will.

This adds a mechanism to detect stalls on the guest vCPUS by creating a
per CPU hrtimer which periodically 'pets' the host backend driver.
On a conventional watchdog-core driver, the userspace is responsible for
delivering the 'pet' events by writing to the particular /dev/watchdogN node.
In this case we require a strong thread affinity to be able to
account for lost time on a per vCPU basis.

This device driver acts as a soft lockup detector by relying on the host
backend driver to measure the elapesed time between subsequent 'pet' events.
If the elapsed time doesn't match an expected value, the backend driver
decides that the guest vCPU is locked and resets the guest. The host
backend driver takes into account the time that the guest is not
running. The communication with the backend driver is done through MMIO
and the register layout of the virtual watchdog is described as part of
the backend driver changes.

The host backend driver is implemented as part of:
https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3548817

Changelog v10:
 - keep only the hrtimer and a flag in the per_cpu structure and move
   the other fields in a separate config structure
 - fix a potential race condition as pointed out by Will: the
   driver remove(..) can race with the hotplug cpu notifiers
 - replace alloc_percpu with devm_alloc_percpu and remove the free_percpu
 - unregister the hotplug notifiers
 - improve the Kconfig description and fix the license in the header
   file
 - add the review-by tag from Rob as the DT has not changed since v9
 
Changelog v9:
 - make the driver depend on CONFIG_OF
 - remove the platform_(set|get)_drvdata calls and keep a per-cpu static
   variable `vm_stall_detect` as suggested by Guenter on the (v8) series
 - improve commit description and fix styling

Sebastian Ene (2):
  dt-bindings: vcpu_stall_detector: Add qemu,vcpu-stall-detector
    compatible
  misc: Add a mechanism to detect stalls on guest vCPUs

 .../misc/qemu,vcpu-stall-detector.yaml        |  51 +++++
 drivers/misc/Kconfig                          |  14 ++
 drivers/misc/Makefile                         |   1 +
 drivers/misc/vcpu_stall_detector.c            | 209 ++++++++++++++++++
 4 files changed, 275 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/misc/qemu,vcpu-stall-detector.yaml
 create mode 100644 drivers/misc/vcpu_stall_detector.c

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2.37.0.rc0.161.g10f37bed90-goog