Re: [PATCH] xen/arm: Make local_events_need_delivery working with idle VPCU

[Ian Campbell <ian.campbell@citrix.com>]

On Mon, 2015-04-27 at 17:32 +0100, Julien Grall wrote:
> >> diff --git a/xen/include/asm-arm/event.h b/xen/include/asm-arm/event.h
> >> index 5330dfe..0149d06 100644
> >> --- a/xen/include/asm-arm/event.h
> >> +++ b/xen/include/asm-arm/event.h
> >> @@ -39,7 +39,12 @@ static inline int local_events_need_delivery_nomask(void)
> >>  
> >>  static inline int local_events_need_delivery(void)
> >>  {
> >> -    if ( !vcpu_event_delivery_is_enabled(current) )
> >> +    struct vcpu *v = current;
> >> +
> >> +    if ( unlikely(is_idle_vcpu(v)) )
> >> +        return 0;
> >> +
> >> +    if ( !vcpu_event_delivery_is_enabled(v) )
> >>          return 0;
> >>      return local_events_need_delivery_nomask();
> >>  }
> > 
> > Is it actually considered correct in Xen to call hypercall_preempt_check
> > and/or local_events_need_delivery on the idle vcpu?
> 
> It seems that the x86 version of hypercall_preempt_check is able to cope
> with idle VCPU. 

AFAICT that's just a coincidence, since an idle vcpu won't ever have a
pending up call.

> Although, I'm not sure if there is path where
> hypercall_preempt_check can be called on an idle VCPU (cc x86
> maintainers for this purpose).
> 
> > Shouldn't it be avoided and maybe a BUG_ON added here instead?
> 
> This patch was the simple way to fix the bug. I have other ideas in mind
> which require some rework in apply_p2m_changes.
> 
> I'll wait to see what x86 maintainers think.

I'm inclined to just go with this patch for now, unless Stefano is
nacking it.

One question first: What aspect of local_events_need_delivery relies on
the vcpu not being an idle one? I suppose something is not initialised,
but what.

Ian.