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

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Message-ID: <20161029080301.5e464435@t450s.home>
Date:   Sat, 29 Oct 2016 08:03:01 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.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 Sat, 29 Oct 2016 16:07:05 +0530
Kirti Wankhede <kwankhede@...dia.com> wrote:

> 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?

That seems like a valid choice.  It's probably better than calling the
notifier for each page of iova.  Thanks,

Alex