Re: [Qemu-devel] [PATCH v10 10/19] vfio iommu: Add blocking notifier to notify DMA_UNMAP

[Dong Jia Shi]

* Kirti Wankhede <address@hidden> [2016-11-01 13:17:19 +0530]:

> 
> 
> On 11/1/2016 9:15 AM, Dong Jia Shi wrote:
> > * Alex Williamson <address@hidden> [2016-10-29 08:03:01 -0600]:
> > 
> >> On Sat, 29 Oct 2016 16:07:05 +0530
> >> Kirti Wankhede <address@hidden> wrote:
> >>
> >>> On 10/29/2016 2:03 AM, Alex Williamson wrote:
> >>>> On Sat, 29 Oct 2016 01:32:35 +0530
> >>>> Kirti Wankhede <address@hidden> wrote:
> >>>>   
> >>>>> On 10/28/2016 6:10 PM, Alex Williamson wrote:  
> >>>>>> On Fri, 28 Oct 2016 15:33:58 +0800
> >>>>>> Jike Song <address@hidden> 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
> >>
> > Hi Kirti,
> > 
> > This version requires the *vendor driver* call vfio_register_notifier
> > for an mdev device before any pinning operations. I guess all of the
> > vendor drivers may have some alike code for notifier
> > registration/unregistration.
> > 
> > My question is, how about letting the mdev framework managing the
> > notifier registration/unregistration process?
> > 
> > We could add a notifier_fn_t callback to "struct parent_ops", then the
> > mdev framework should make sure that the vendor driver assigned a value
> > to this callback. The mdev core could initiate a notifier_block for each
> > parent driver with its callback, and register/unregister it to vfio in
> > the right time.
> > 
> 
> Module mdev_core is independent of VFIO so far and it should be
> independent of VFIO module.
> 
> Its a good suggestion to have a notifier callback in parent_ops, but we
> shouldn't call vfio_register_notifier()/ vfio_unregister_notifier() from
> mdev core module. vfio_mdev module would take care to
> register/unregister notifier to vfio from its vfio_mdev_open()/
> vfio_mdev_release() call. That looks cleaner to me.
Nod.

> 
> Notifier callback in parent_ops should be optional since all vendor
> drivers might not pin/unpin pages, for example sample mtty driver.
> Notifier would be registered to vfio module only if the notifier
> callback is provided by vendor driver.
> 
> If this looks reasonable, I'll have this patch in my next version of
> patch series.
Sounds reasonable for me. Thanks,

> 
> Thanks,
> Kirti
> 

-- 
Dong Jia