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
