[RFC PATCH 0/2] arm64/hyperv: Enable kexec reboot support

[Shradha Gupta <shradhagupta@linux.microsoft.com>]

Kexec reboot consistently fails on ARM64 Hyper-V guests (Azure VMs).
During the kexec shutdown path, VMBus channels are never cleaned up,
so in the fresh kexec kernel startup hv_acpi_init() blocks because
the hypervisor still holds the old kernel's VMBus session open.

This is because ARM64 lacks the VMBus teardown that x86 performs during
kexec via hv_machine_shutdown(). On x86, machine_ops.shutdown is
overridden to send CHANNELMSG_UNLOAD and disable SynIC before CPUs
go offline. ARM64 has no equivalent mechanism.

Why existing notification mechanisms don't work:

- Reboot notifiers: Fire too early - they run inside
  kernel_restart_prepare(), BEFORE device_shutdown(). VMBus UNLOAD
  must happen AFTER device_shutdown() so that PCI drivers can clean
  up interrupt mappings (PCI_DELETE_INTERRUPT_MESSAGE) before UNLOAD
  force-closes channels. Sending UNLOAD first causes interrupt
  mapping leaks because the mappings aren't released on channel
  close.

- Device .shutdown callbacks: Too narrow - these handle per-device
  cleanup (e.g., individual VMBus channel teardown), but cannot
  perform bus-level operations like sending the global VMBus UNLOAD
  message or removing the SynIC CPU hotplug state via
  cpuhp_remove_state().

- VMBus parent device .shutdown callback: Also insufficient -
  cpuhp_remove_state() for SynIC teardown must run after all device
  shutdown completes, not during it. A parent .shutdown callback
  still executes within device_shutdown().

- The required window is: after device_shutdown() completes, after
  cpu_hotplug_enable(), but before smp_shutdown_nonboot_cpus(). That
  window exists inside machine_shutdown(), which currently has no hook.

The need for an ARM64 shutdown hook was previously discussed in [1]
but lacked a concrete failure case at the time. We now have one.

[1] https://lore.kernel.org/linux-arm-kernel/427a8277-49f0-4317-d6c3-4a15d7070e55@igalia.com/

This RFC proposes fixing kexec on ARM64 Hyper-V guests with:

Patch 1: A platform hook (arm64_pre_smp_shutdown_hook) in ARM64's
  machine_shutdown(), analogous to x86's machine_ops.shutdown. This
  runs after device_shutdown() and cpu_hotplug_enable(), allowing
  platform code to inject pre-shutdown logic at the right point in
  the kexec path.

  Design choices I'd like feedback on:
  - Single function pointer vs full machine_ops struct: ARM64 uses
    kernel-wide APIs (register_restart_handler, register_platform_power_off)
    for restart/poweroff rather than x86's monolithic machine_ops, so
    a targeted hook seemed more consistent with the ARM64 pattern.
  - On ARM64, machine_shutdown() is only called from kernel_kexec(),
    unlike x86/powerpc where it's also called from restart/halt/poweroff.

Patch 2: Uses this hook to call hv_kexec_handler() which performs:
  - vmbus_initiate_unload(): sends CHANNELMSG_UNLOAD to host
  - cpuhp_remove_state(): disables SynIC (SIMP, SIEFP, SINT) on
    all CPUs, ensuring the kexec'd kernel starts with clean state

Tested on ARM64 Azure VMs (Ubuntu 22.04, multiple vCPU configs):
- kexec reboot succeeds, VM comes back online with SSH
- Normal reboot/poweroff unaffected
- Multiple consecutive kexec cycles pass

Looking for feedback on:
1. Is the single function pointer hook acceptable for ARM64, or would
   the ARM64 maintainers prefer a registration API or __weak function?
2. Should this be arm64-specific or generic kexec infrastructure?

Shradha Gupta (2):
  arm64: Add pre-shutdown hook to machine_shutdown()
  arm64/hyperv: Add kexec handler using machine_shutdown hook

 arch/arm64/hyperv/mshyperv.c         | 37 ++++++++++++++++++++++++++++
 arch/arm64/include/asm/system_misc.h |  2 ++
 arch/arm64/kernel/process.c          | 13 ++++++++++
 3 files changed, 52 insertions(+)

-- 
2.43.0