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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 27 Apr 2015, Julien Grall wrote:
> The commit 569fb6c "xen/arm: Data abort exception (R/W) mem_access
> events" makes apply_p2m_changes to call hypercall_preempt_check for any
> operation rather than for relinquish.
> 
> The function hypercall_preempt_check call local_events_need_delivery
> which rely on the current VCPU is not an idle VCPU.
> Although, during DOM0 building the current VCPU is an idle one. This
> would make Xen crash with the following stack trace:
> 
> (XEN) CPU0: Unexpected Trap: Data Abort
> [...]
> (XEN) Xen call trace:
> (XEN)    [<00256ef4>] apply_p2m_changes+0x210/0x1190 (PC)
> (XEN)    [<002506b4>] gic_events_need_delivery+0x5c/0x13c (LR)
> (XEN)    [<002580ec>] map_mmio_regions+0x64/0x74
> (XEN)    [<00251958>] gicv2v_setup+0xf8/0x150
> (XEN)    [<00250964>] gicv_setup+0x20/0x30
> (XEN)    [<0024cb3c>] arch_domain_create+0x170/0x244
> (XEN)    [<00207df0>] domain_create+0x2ac/0x4d8
> (XEN)    [<0028e3d0>] start_xen+0xcbc/0xee4
> (XEN)    [<00200540>] paging+0x94/0xd8
> (XEN)
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 0:
> (XEN) CPU0: Unexpected Trap: Data Abort
> (XEN)
> (XEN) ****************************************
> 
> As an idle VCPU can never receive an event, return 0 when the current
> VCPU is an idle VCPU in local_events_need_delivery.
> 
> Reported-by: Riku Voipio <riku.voipio@xxxxxxxxxx>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> CC: Tamas K Lengyel <tklengyel@xxxxxxxxxxxxx>
> 
> ---
> 
> This bug has been catched during boot on Mustang. This is because we
> have to map large chunk of PCI memory region.
> 
> I was able to reproduce the bug on midway by lowering down
> preempt_count_limit to 16 in apply_p2m_changes.
> 
> Although, I'm not sure this is the right fix for the bug.
> ---
>  xen/include/asm-arm/event.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> 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?

Shouldn't it be avoided and maybe a BUG_ON added here instead?

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