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

[Dong Jia Shi]

* 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.

-- 
Dong Jia