[RFC PATCH 1/2] arm64: Add pre-shutdown hook to machine_shutdown()

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

Add a function pointer hook (arm64_pre_smp_shutdown_hook) that is invoked
from machine_shutdown() before smp_shutdown_nonboot_cpus(). This allows
platform code (e.g., hypervisors) to perform cleanup that must happen
after device_shutdown() but before secondary CPUs go offline.

In the kexec path, the call sequence is:
  kernel_kexec()
    kernel_restart_prepare()
      device_shutdown()         // drivers shut down here
    migrate_to_reboot_cpu()
    cpu_hotplug_enable()
    machine_shutdown()
      arm64_pre_smp_shutdown_hook()  // new: platform cleanup
      smp_shutdown_nonboot_cpus()    // CPUs go offline

x86 achieves this via machine_ops.shutdown; ARM64 currently has no
equivalent mechanism. Rather than introducing the full machine_ops
structure, add a targeted hook for the shutdown path.

Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
---
 arch/arm64/include/asm/system_misc.h |  2 ++
 arch/arm64/kernel/process.c          | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/arch/arm64/include/asm/system_misc.h b/arch/arm64/include/asm/system_misc.h
index d316a804eb38..7c37b9f33e96 100644
--- a/arch/arm64/include/asm/system_misc.h
+++ b/arch/arm64/include/asm/system_misc.h
@@ -28,6 +28,8 @@ void arm64_notify_die(const char *str, struct pt_regs *regs,
 struct mm_struct;
 extern void __show_regs(struct pt_regs *);
 
+extern void (*arm64_pre_smp_shutdown_hook)(void);
+
 #endif	/* __ASSEMBLER__ */
 
 #endif	/* __ASM_SYSTEM_MISC_H */
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 581f80e9b9b7..41c0e9840913 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -88,8 +88,21 @@ void __noreturn arch_cpu_idle_dead(void)
  * avoid any code or data used by any SW CPU pin loop. The CPU hotplug
  * functionality embodied in smpt_shutdown_nonboot_cpus() to achieve this.
  */
+
+/*
+ * Hook for platform code to perform cleanup after device_shutdown()
+ * but before secondary CPUs are offlined. This runs in the kexec path
+ * from kernel_kexec() after device_shutdown() and cpu_hotplug_enable()
+ * have been called, matching the point at which x86 invokes
+ * machine_ops.shutdown.
+ */
+void (*arm64_pre_smp_shutdown_hook)(void);
+
 void machine_shutdown(void)
 {
+	if (arm64_pre_smp_shutdown_hook)
+		arm64_pre_smp_shutdown_hook();
+
 	smp_shutdown_nonboot_cpus(reboot_cpu);
 }
 
-- 
2.43.0