[PATCH v2 2/2] x86/xen: Make the secondary CPU idle tasks reliable

[Miroslav Benes]

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.

Introduce a wrapper in assembly, which just calls
cpu_bringup_and_idle(). The return address is thus pushed on the stack
and the wrapper contains the annotation hint for the unwinder regarding
the stack state.

Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
---
 arch/x86/xen/smp_pv.c   | 3 ++-
 arch/x86/xen/xen-head.S | 8 ++++++++
 2 files changed, 10 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)
 {
@@ -309,7 +310,7 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
 	 * pointing just below where pt_regs would be if it were a normal
 	 * kernel entry.
 	 */
-	ctxt->user_regs.eip = (unsigned long)cpu_bringup_and_idle;
+	ctxt->user_regs.eip = (unsigned long)asm_cpu_bringup_and_idle;
 	ctxt->flags = VGCF_IN_KERNEL;
 	ctxt->user_regs.eflags = 0x1000; /* IOPL_RING1 */
 	ctxt->user_regs.ds = __USER_DS;
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index edc776af0e0a..9dc6f9a420a8 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -58,6 +58,14 @@ SYM_CODE_START(startup_xen)
 	call xen_start_kernel
 SYM_CODE_END(startup_xen)
 	__FINIT
+
+.pushsection .text
+SYM_CODE_START(asm_cpu_bringup_and_idle)
+	UNWIND_HINT_EMPTY
+
+	call cpu_bringup_and_idle
+SYM_CODE_END(asm_cpu_bringup_and_idle)
+.popsection
 #endif
 
 .pushsection .text
-- 
2.25.1