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

[Shradha Gupta]

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