Re: Multiple issues with event channel on Xen on ARM

[David Vrabel <david.vrabel@citrix.com>]

On 04/02/14 23:18, Julien Grall wrote:
> Hello David,
> 
> I'm currently trying to use Linux 3.14-rc1 as Linux guest on Xen on ARM (Xen 4.4-rc3).
> 
> I have multiple issues with your event channel patch series on Linux and Xen side.
> I tried to use Linux 3.14-rc1 as dom0 but it was worst (unable to create guests).

I think there must be two issues here as both 2-level and FIFO events
are broken.

> I'm using a simple guest config:
> kernel="/root/zImage"
> memory=32
> name="test"
> vcpus=1
> autoballon="off"
> extra="console=hvc0"
> 
> If everything is ok, I should see that Linux is unable to find the root filesystem.
> But here, Linux is stucked.
> 
>>From Linux side, after bisecting, I found that the offending commit is:
>     xen/events: remove unnecessary init_evtchn_cpu_bindings()
>     
>     Because the guest-side binding of an event to a VCPU (i.e., setting
>     the local per-cpu masks) is always explicitly done after an event
>     channel is bound to a port, there is no need to initialize all
>     possible events as bound to VCPU 0 at start of day or after a resume.
>     
>     Signed-off-by: David Vrabel <david.vrabel@citrix.com>
>     Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>     Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> 
> With this patch, the function __xen_evtchn_do_upcall won't be able
> to find an events (pendings_bits == 0 every time).
> It seems the second part of init_evtchn_cpu_bindings is necessary on ARM.

I think this is because binding an interdomain or allocating an unbound
event channel does call bind_evtchn_to_cpu(evtchn, 0) which is required
to set the local VCPU masks.

I think this happened to work on x86 because during the generic irq
setup, the irq affinity is always set which then binds the event channel
to the right VCPU.  I guess ARM's irq setup misses this step.

This shouldn't affect the FIFO-based events though since
evtchn_fifo_bind_to_cpu() is a no-op.

David