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

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Date:	Tue, 14 Jul 2015 17:07:09 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	Joerg Roedel <jroedel@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	xiao jin <jin.xiao@...el.com>, Peter Anvin <hpa@...or.com>,
	xen-devel <xen-devel@...ts.xenproject.org>,
	Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [Xen-devel] [patch 1/4] hotplug: Prevent alloc/free of irq descriptors
 during cpu up/down

On 07/14/2015 04:15 PM, Thomas Gleixner wrote:
> On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
>> 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().
> I think we should revisit this for 4.3. For 4.2 we can do the trivial
> variant and move the locking in native_cpu_up() and x86 only. x86 was
> the only arch on which such wreckage has been seen in the wild, but we
> should have that protection for all archs in the long run.
>
> Patch below should fix the issue.


Thanks! Most of my tests passed, I had a couple of failures but I will 
need to see whether they are related to this patch.

-boris
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