linux-kernel - Re: [PATCH v11 11/22] vfio iommu: Add blocking notifier to notify DMA

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Date:   Tue, 8 Nov 2016 21:56:29 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     <pbonzini@...hat.com>, <kraxel@...hat.com>, <cjia@...dia.com>,
        <qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
        <kevin.tian@...el.com>, <jike.song@...el.com>,
        <bjsdjshi@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 11/22] vfio iommu: Add blocking notifier to notify
 DMA_UNMAP



On 11/8/2016 5:15 AM, Alex Williamson wrote:
> On Sat, 5 Nov 2016 02:40:45 +0530
> Kirti Wankhede <kwankhede@...dia.com> wrote:
> 
...
>>  
>> +int vfio_register_notifier(struct device *dev, struct notifier_block *nb)
> 
> Is the expectation here that this is a generic notifier for all
> vfio->mdev signaling?  That should probably be made clear in the mdev
> API to avoid vendor drivers assuming their notifier callback only
> occurs for unmaps, even if that's currently the case.
> 

Ok. Adding comment about notifier callback in mdev_device which is part
of next patch.

...

>>  	mutex_lock(&iommu->lock);
>>  
>> -	if (!iommu->external_domain) {
>> +	/* Fail if notifier list is empty */
>> +	if ((!iommu->external_domain) || (!iommu->notifier.head)) {
>>  		ret = -EINVAL;
>>  		goto pin_done;
>>  	}
>> @@ -867,6 +870,11 @@ unlock:
>>  	/* Report how much was unmapped */
>>  	unmap->size = unmapped;
>>  
>> +	if (unmapped && iommu->external_domain)
>> +		blocking_notifier_call_chain(&iommu->notifier,
>> +					     VFIO_IOMMU_NOTIFY_DMA_UNMAP,
>> +					     unmap);
> 
> This is after the fact, there's already a gap here where pages are
> unpinned and the mdev device is still running.

Oh, there is a bug here, now unpin_pages() take user_pfn as argument and
find vfio_dma. If its not found, it doesn't unpin pages. We have to call
this notifier before vfio_remove_dma(). But if we call this before
vfio_remove_dma() there will be deadlock since iommu->lock is already
held here and vfio_iommu_type1_unpin_pages() will also try to hold
iommu->lock.
If we want to call blocking_notifier_call_chain() before
vfio_remove_dma(), sequence should be:

unmapped += dma->size;
mutex_unlock(&iommu->lock);
if (iommu->external_domain)) {
	struct vfio_iommu_type1_dma_unmap nb_unmap;

	nb_unmap.iova = dma->iova;
	nb_unmap.size = dma->size;
	blocking_notifier_call_chain(&iommu->notifier,
        	                     VFIO_IOMMU_NOTIFY_DMA_UNMAP,
                	             &nb_unmap);
}
mutex_lock(&iommu->lock);
vfio_remove_dma(iommu, dma);


>  The notifier needs to
> happen prior to that and I suspect that we need to validate that we
> have no remaining external pfn references within this vfio_dma block.
> It seems like we need to root our pfn tracking in the vfio_dma so that
> we can see that it's empty after the notifier chain and BUG_ON if not.

There is no way to find pfns from that iova range with current
implementation. We can have this validate if we go with linear array of
iova to track pfns.

> I would also add some enforcement that external pinning is only enabled
> when vfio_iommu_type1 is configured for v2 semantics (ie. we only
> support unmaps exactly matching previous maps).
> 

Ok I'll add that check.

Thanks,
Kirti