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

[Miroslav Benes]

On Fri, 13 Mar 2020, JÃrgen Groà wrote:

> On 12.03.20 15:20, Miroslav Benes wrote:
> > 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@xxxxxxx>
> > ---
> >   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)
> >   {
> 
> Would adding this here work?
> 
> +	asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));

I tried something similar. It did not work, because than the hint is 
"bound" to the closest next call in the function which is cr4_init() in 
this case. The unwinder would not take it into account.

In my case, I placed it at the beginning of cpu_bringup_and_idle(). I also 
open coded it and played with the offset in the orc entry, but that did 
not work for some other reason.

However, now I tried this

diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 6b88cdcbef8f..39afd88309cb 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
 {
        cpu_bringup();
        boot_init_stack_canary();
+       asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));
        cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
 }

and that seems to work. I need to properly verify and test, but the 
explanation is that as opposed to the above, cpu_startup_entry() is on the 
idle task's stack and the hint is then taken into account. The unwound 
stack seems to be complete, so it could indeed be the fix.

Thanks
Miroslav