[BUG] x86/virt/tdx: tdx_offline_cpu() violates tdx_cpu_flush_cache() preemption assert

[David CARLIER]

Hi,

  In commit 597bdf6e068e ("x86/virt/tdx: Pull kexec cache flush logic into
  arch/x86"), tdx_offline_cpu() gained a call to tdx_cpu_flush_cache(),
  which starts with lockdep_assert_preemption_disabled().

  tdx_offline_cpu() is registered at CPUHP_AP_ONLINE_DYN. ONLINE-section
  teardown callbacks run from the pinned per-CPU hotplug thread with
  preemption and interrupts enabled (Documentation/core-api/cpu_hotplug.rst,
  and cpuhp_thread_fun() only disables IRQs for atomic states).

  The other callers — tdx_shutdown_cpu() via on_each_cpu(), and the
  crash path — satisfy the assertion. Only the offline path doesn't, and
  the splat should fire on every offline once the TDX module is
  initialized and the done: path is taken.

  Wrapping the call with preempt_disable() / preempt_enable() at the
  offline site keeps the contract for the kexec/shutdown callers.

  Not yet reproduced on a debug kernel; reporting on inspection.

  Fixes: 597bdf6e068e ("x86/virt/tdx: Pull kexec cache flush logic
into arch/x86")

  Cheers,
  David