linux-kernel - Re: [PATCH 2/2] powerpc/iommu: check dev->iommu_group before remove a device from iommu

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Message-ID: <20130822154107.GC7393@weiyang.vnet.ibm.com>
Date:	Thu, 22 Aug 2013 23:41:07 +0800
From:	Wei Yang <weiyang@...ux.vnet.ibm.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Alexey Kardashevskiy <aik@...abs.ru>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	benh@....ibm.com, paulus@....ibm.com
Subject: Re: [PATCH 2/2] powerpc/iommu: check dev->iommu_group before remove
 a device from iommu_group

On Thu, Aug 22, 2013 at 09:28:23AM -0600, Alex Williamson wrote:
>On Thu, 2013-08-22 at 15:52 +0800, Wei Yang wrote:
>> On Thu, Aug 22, 2013 at 05:23:34PM +1000, Alexey Kardashevskiy wrote:
>> >On 08/19/2013 11:55 AM, Wei Yang wrote:
>> >> On Mon, Aug 19, 2013 at 11:39:49AM +1000, Alexey Kardashevskiy wrote:
>> >>> On 08/19/2013 11:29 AM, Wei Yang wrote:
>> >>>> On Fri, Aug 16, 2013 at 08:15:36PM +1000, Alexey Kardashevskiy wrote:
>> >>>>> On 08/16/2013 08:08 PM, Wei Yang wrote:
>> >>>>>> ---
>> >>>>>>  arch/powerpc/kernel/iommu.c |    3 ++-
>> >>>>>>  1 files changed, 2 insertions(+), 1 deletions(-)
>> >>>>>>
>> >>>>>> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
>> >>>>>> index b20ff17..5abf7c3 100644
>> >>>>>> --- a/arch/powerpc/kernel/iommu.c
>> >>>>>> +++ b/arch/powerpc/kernel/iommu.c
>> >>>>>> @@ -1149,7 +1149,8 @@ static int iommu_bus_notifier(struct notifier_block *nb,
>> >>>>>>  	case BUS_NOTIFY_ADD_DEVICE:
>> >>>>>>  		return iommu_add_device(dev);
>> >>>>>>  	case BUS_NOTIFY_DEL_DEVICE:
>> >>>>>> -		iommu_del_device(dev);
>> >>>>>> +		if (dev->iommu_group)
>> >>>>>> +			iommu_del_device(dev);
>> >>>>>>  		return 0;
>> >>>>>>  	default:
>> >>>>>>  		return 0;
>> >>>>>>
>> >>>>>
>> >>>>> This one seems redundant, no?
>> >>>>
>> >>>> Sorry for the late.
>> >>>>
>> >>>> Yes, these two patches have the same purpose to guard the system, while in two
>> >>>> different places.  One is in powernv platform, the other is in the generic iommu 
>> >>>> driver.
>> >>>>
>> >>>> The one in powernv platform is used to correct the original logic.
>> >>>>
>> >>>> The one in generic iommu driver is to keep system safe in case other platform to
>> >>>> call iommu_group_remove_device() without the check.
>> >>>
>> >>>
>> >>> But I am moving bus notifier to powernv code (posted a patch last week,
>> >>> otherwise Freescale's IOMMU conflicted) so this won't be the case.
>> >> 
>> >> Yes, I see the patch.
>> >> 
>> >> This means other platforms, besides powernv, will check the dev->iommu_group
>> >> before remove the device? This would be a convention?
>> >> 
>> >> If this is the case, the second patch is enough. We don't need to check it in
>> >> generic iommu driver.
>> >> 
>> >> Since I am not very familiar with the code convention, I post these two
>> >> patches together. This doesn't mean I need to push both of them. Your comments
>> >> are welcome, lets me understand which one is more suitable in this case.
>> >
>> >
>> >Ok. So. I included the check in the bus notifier which I moved to powernv
>> >platform, I guess I'll repost the series soon.
>> 
>> Thanks, this check will guard the powernv platform.
>> 
>> >
>> >Good luck with pushing the fix for drivers/iommu/iommu.c :)
>> >
>> 
>> Alex,
>> 
>> Sorry for not including you in the very beginning, which may spend you more
>> efforts to track previous mails in this thread.
>> 
>> Do you think it is reasonable to check the dev->iommu_group in
>> iommu_group_remove_device()? Or we can count on the bus notifier to check it?
>> 
>> Welcome your suggestions~
>
>I don't really see the point of patch 1/2. iommu_group_remove_device()
>is specifically to remove a device from an iommu_group, so why would you
>call it on a device that's not part of an iommu_group.  If you want to
>avoid testing dev->iommu_group, then implement the .remove_device
>callback rather than using the notifier.  Thanks,
>

You mean the .remove_device like intel_iommu_remove_device()? 

Hmm... this function didn't check the dev->iommu_group and just call
iommu_group_remove_device(). I see this guard is put in iommu_bus_notifier(), 
which will check dev->iommu_group before invoke .remove_device.

Let me explain the case to triger the problem a little. 

On some platform, like powernv, we implement another bus notifier when devices
are added or removed in the system. Like Alexey mentioned, he missed the check
for dev->iommu_group in the notifier before removing it from iommu_group. This
trigger the crash.

So do you think it is reasonable to guard the kernel in
iommu_group_remove_device(), or we give the platform developers the
responsibility to check the dev->iommu_group before calling it?

Thanks~

>Alex

-- 
Richard Yang
Help you, Help me

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