The unwinder reports the secondary CPU idle tasks' stack on XEN PV as
unreliable, which affects at least live patching.
cpu_initialize_context() sets up the context of the CPU through
VCPUOP_initialise hypercall. After it is woken up, the idle task starts
in cpu_bringup_and_idle() function and its stack starts at the offset
right below pt_regs. The unwinder correctly detects the end of stack
there but it is confused by NULL return address in the last frame.

RFC: I haven't found the way to teach the unwinder about the state of
the stack there. Thus the ugly hack using assembly. Similar to what
startup_xen() has got for boot CPU.

It introduces objtool "unreachable instruction" warning just right after
the jump to cpu_bringup_and_idle(). It should show the idea what needs
to be done though, I think. Ideas welcome.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
---
  arch/x86/xen/smp_pv.c   |  3 ++-
  arch/x86/xen/xen-head.S | 10 ++++++++++
  2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 802ee5bba66c..6b88cdcbef8f 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c

@@ -53,6 +53,7 @@ static DEFINE_PER_CPU(struct xen_common_irq, xen_irq_work) = { .irq = -1 };
  static DEFINE_PER_CPU(struct xen_common_irq, xen_pmu_irq) = { .irq = -1 };
  
  static irqreturn_t xen_irq_work_interrupt(int irq, void *dev_id);
+extern unsigned char asm_cpu_bringup_and_idle[];
  
  static void cpu_bringup(void)
  {