Re: [Xen-devel] [patch 1/4] hotplug: Prevent alloc/free of irq descriptors during cpu up/down

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 07/14/2015 01:32 PM, Thomas Gleixner wrote:
> On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
>> On 07/14/2015 11:44 AM, Thomas Gleixner wrote:
>>> On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
>>>>> Prevent allocation and freeing of interrupt descriptors accross cpu
>>>>> hotplug.
>>>> This breaks Xen guests that allocate interrupt descriptors in .cpu_up().
>>> And where exactly does XEN allocate those descriptors?
>> xen_cpu_up()
>>      xen_setup_timer()
>>          bind_virq_to_irqhandler()
>>              bind_virq_to_irq()
>>                  xen_allocate_irq_dynamic()
>>                      xen_allocate_irqs_dynamic()
>>                          irq_alloc_descs()
>>
>>
>> There is also a similar pass via xen_cpu_up() -> xen_smp_intr_init()
> Sigh.
>   
>>>    
>>>> Any chance this locking can be moved into arch code?
>>> No.
> The issue here is that all architectures need that protection and just
> Xen does irq allocations in cpu_up.
>
> So moving that protection into architecture code is not really an
> option.
>
>>>> Otherwise we will need to have something like arch_post_cpu_up()
>>>> after the lock is released.
> I'm not sure, that this will work. You probably want to do this in the
> cpu prepare stage, i.e. before calling __cpu_up().



For PV guests (the ones that use xen_cpu_up()) it will work either 
before or after __cpu_up(). At least my (somewhat limited) testing 
didn't show any problems so far.

However, HVM CPUs use xen_hvm_cpu_up() and if you read comments there 
you will see that xen_smp_intr_init() needs to be called before 
native_cpu_up() but xen_init_lock_cpu() (which eventually calls 
irq_alloc_descs()) needs to be called after.

I think I can split xen_init_lock_cpu() so that the part that needs to 
be called after will avoid going into irq core code. And then the rest 
will go into arch_cpu_prepare().


-boris