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Subject[RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks reliable
Date
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)
{
@@ -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 642f346bfe02..c9a9c0bb79ed 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -56,6 +56,16 @@ SYM_CODE_START(startup_xen)
1:
SYM_CODE_END(startup_xen)
__FINIT
+
+.pushsection .text
+SYM_CODE_START(asm_cpu_bringup_and_idle)
+ UNWIND_HINT_EMPTY
+
+ push $1f
+ jmp cpu_bringup_and_idle
+1:
+SYM_CODE_END(asm_cpu_bringup_and_idle)
+.popsection
#endif

.pushsection .text
--
2.25.1
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