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

On Fri, Aug 14, 2026 at 02:31:28AM -0700, Shradha Gupta wrote:

Gentle Reminder

Regards,
Shradha.

> 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/[email protected]/
> 
> 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