On Fri, Aug 21, 2026 at 11:14:49AM +0100, Catalin Marinas wrote: > On Fri, Aug 14, 2026 at 02:31:28AM -0700, Shradha Gupta wrote: > > 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. > > I'm really not keen on such hook, a bare pointer you set elsewhere. Not > sure it was discussed before but can you not use the syscore_shutdown() > mechanism? This hook got added in 2023, so the 2022 discussion you > referenced precedes it. > > -- > Catalin
Thanks for the suggestion, Catalin. I looked into syscore_shutdown() and it should work. The syscore_shutdown() call in the kexec path runs after device_shutdown() and before smp_shutdown_nonboot_cpus(), which is exactly the window where VMBus teardown needs to happen. It does run before cpu_hotplug_enable() (unlike the x86 path where hv_kexec_handler runs inside machine_shutdown, after cpu_hotplug_enable), but that should be safe because cpuhp_remove_state() acquires cpus_read_lock, which should be independent of the cpu_hotplug_disabled flag. The implementation would override hv_setup_kexec_handler() in arch/arm64/hyperv/mshyperv.c to register a syscore_ops.shutdown callback, guarded by kexec_in_progress. No ARM64 arch changes needed, and other shutdown paths (reboot, halt, poweroff, kdump) should also be unaffected. I am testing this now and will send a v2 with the updated approach. Shradha
