linux-kernel - Re: [PATCH v10 10/19] vfio iommu: Add blocking notifier to notify DMA

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Date:   Sat, 29 Oct 2016 16:07:05 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Jike Song <jike.song@...el.com>, <pbonzini@...hat.com>,
        <kraxel@...hat.com>, <cjia@...dia.com>, <qemu-devel@...gnu.org>,
        <kvm@...r.kernel.org>, <kevin.tian@...el.com>,
        <bjsdjshi@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v10 10/19] vfio iommu: Add blocking notifier to notify
 DMA_UNMAP



On 10/29/2016 2:03 AM, Alex Williamson wrote:
> On Sat, 29 Oct 2016 01:32:35 +0530
> Kirti Wankhede <kwankhede@...dia.com> wrote:
> 
>> On 10/28/2016 6:10 PM, Alex Williamson wrote:
>>> On Fri, 28 Oct 2016 15:33:58 +0800
>>> Jike Song <jike.song@...el.com> wrote:
>>>   
...
>>>>>  
>>>>> +/*
>>>>> + * This function finds pfn in domain->external_addr_space->pfn_list for given
>>>>> + * iova range. If pfn exist, notify pfn to registered notifier list. On
>>>>> + * receiving notifier callback, vendor driver should invalidate the mapping and
>>>>> + * call vfio_unpin_pages() to unpin this pfn. With that vfio_pfn for this pfn
>>>>> + * gets removed from rb tree of pfn_list. That re-arranges rb tree, so while
>>>>> + * searching for next vfio_pfn in rb tree, start search from first node again.
>>>>> + * If any vendor driver doesn't unpin that pfn, vfio_pfn would not get removed
>>>>> + * from rb tree and so in next search vfio_pfn would be same as previous
>>>>> + * vfio_pfn. In that case, exit from loop.
>>>>> + */
>>>>> +static void vfio_notifier_call_chain(struct vfio_iommu *iommu,
>>>>> +				     struct vfio_iommu_type1_dma_unmap *unmap)
>>>>> +{
>>>>> +	struct vfio_domain *domain = iommu->external_domain;
>>>>> +	struct rb_node *n;
>>>>> +	struct vfio_pfn *vpfn = NULL, *prev_vpfn;
>>>>> +
>>>>> +	do {
>>>>> +		prev_vpfn = vpfn;
>>>>> +		mutex_lock(&domain->external_addr_space->pfn_list_lock);
>>>>> +
>>>>> +		n = rb_first(&domain->external_addr_space->pfn_list);
>>>>> +
>>>>> +		for (; n; n = rb_next(n), vpfn = NULL) {
>>>>> +			vpfn = rb_entry(n, struct vfio_pfn, node);
>>>>> +
>>>>> +			if ((vpfn->iova >= unmap->iova) &&
>>>>> +			    (vpfn->iova < unmap->iova + unmap->size))
>>>>> +				break;
>>>>> +		}
>>>>> +
>>>>> +		mutex_unlock(&domain->external_addr_space->pfn_list_lock);
>>>>> +
>>>>> +		/* Notify any listeners about DMA_UNMAP */
>>>>> +		if (vpfn)
>>>>> +			blocking_notifier_call_chain(&iommu->notifier,
>>>>> +						    VFIO_IOMMU_NOTIFY_DMA_UNMAP,
>>>>> +						    &vpfn->pfn);    
>>>>
>>>> Hi Kirti, 
>>>>
>>>> The information carried by notifier is only a pfn.
>>>>
>>>> Since your pin/unpin interfaces design, it's the vendor driver who should
>>>> guarantee pin/unpin same times. To achieve that, the vendor driver must
>>>> cache it's iova->pfn mapping on its side, to avoid pinning a same page
>>>> for multiple times.
>>>>
>>>> With the notifier carrying only a pfn, to find the iova by this pfn,
>>>> the vendor driver must *also* keep a reverse-mapping. That's a bit
>>>> too much.
>>>>
>>>> Since the vendor could also suffer from IOMMU-compatible problem,
>>>> which means a local cache is always helpful, so I'd like to have the
>>>> iova carried to the notifier.
>>>>
>>>> What'd you say?  
>>>
>>> I agree, the pfn is not unique, multiple guest pfns (iovas) might be
>>> backed by the same host pfn.  DMA_UNMAP calls are based on iova, the
>>> notifier through to the vendor driver must be based on the same.  
>>
>> Host pfn should be unique, right?
> 
> Let's say a user does a malloc of a single page and does 100 calls to
> MAP_DMA populating 100 pages of IOVA space all backed by the same
> malloc'd page.  This is valid, I have unit tests that do essentially
> this.  Those will all have the same pfn.  The user then does an
> UNMAP_DMA to a single one of those IOVA pages.  Did the user unmap
> everything matching that pfn?  Of course not, they only unmapped that
> one IOVA page.  There is no guarantee of a 1:1 mapping of pfn to IOVA.
> UNMAP_DMA works based on IOVA.  Invalidation broadcasts to the vendor
> driver MUST therefore also work based on IOVA.  This is not an academic
> problem, address space aliases exist in real VMs, imagine a virtual
> IOMMU.  Thanks,
> 


So struct vfio_iommu_type1_dma_unmap should be passed as argument to
notifier callback:

        if (unmapped && iommu->external_domain)
-               vfio_notifier_call_chain(iommu, unmap);
+               blocking_notifier_call_chain(&iommu->notifier,
+                                           VFIO_IOMMU_NOTIFY_DMA_UNMAP,
+                                            unmap);

Then vendor driver should find pfns he has pinned from this range of
iovas, then invalidate and unpin pfns. Right?

Thanks,
Kirti