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

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Message-ID: <1377188240.25163.23.camel@ul30vt.home>
Date:	Thu, 22 Aug 2013 10:17:20 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Wei Yang <weiyang@...ux.vnet.ibm.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, 2013-08-22 at 23:41 +0800, Wei Yang wrote:
> 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?

I don't see it as we need either patch 1/2 or patch 2/2.  We absolutely
need some form of patch 2/2.  Patch 1/2 isn't necessarily bad, but it
facilitates sloppy usage.  The iommu driver shouldn't be calling
iommu_group_remove_device() on arbitrary devices that may or may not be
part of an iommu_group.  Perhaps patch 1/2 should be:

if (WARN_ON(!group))
	return;

Thanks,

Alex

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