Re: [Qemu-devel] [PATCHv3 6/7] vfio: Allow hotplug of containers onto existing guest IOMMU mappings

[David Gibson]

On Wed, Sep 30, 2015 at 11:09:17AM +0200, Laurent Vivier wrote:
> 
> 
> On 30/09/2015 04:13, David Gibson wrote:
> > At present the memory listener used by vfio to keep host IOMMU mappings
> > in sync with the guest memory image assumes that if a guest IOMMU
> > appears, then it has no existing mappings.
> > 
> > This may not be true if a VFIO device is hotplugged onto a guest bus
> > which didn't previously include a VFIO device, and which has existing
> > guest IOMMU mappings.
> > 
> > Therefore, use the memory_region_register_iommu_notifier_replay()
> > function in order to fix this case, replaying existing guest IOMMU
> > mappings, bringing the host IOMMU into sync with the guest IOMMU.
> > 
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  hw/vfio/common.c | 23 +++++++++--------------
> >  1 file changed, 9 insertions(+), 14 deletions(-)
> > 
> > diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> > index f666de2..6797208 100644
> > --- a/hw/vfio/common.c
> > +++ b/hw/vfio/common.c
> > @@ -312,6 +312,11 @@ out:
> >      rcu_read_unlock();
> >  }
> >  
> > +static hwaddr vfio_container_granularity(VFIOContainer *container)
> > +{
> > +    return (hwaddr)1 << ctz64(container->iova_pgsizes);
> > +}
> > +
> >  static void vfio_listener_region_add(MemoryListener *listener,
> >                                       MemoryRegionSection *section)
> >  {
> > @@ -369,26 +374,16 @@ static void vfio_listener_region_add(MemoryListener 
> > *listener,
> >           * would be the right place to wire that up (tell the KVM
> >           * device emulation the VFIO iommu handles to use).
> >           */
> > -        /*
> > -         * This assumes that the guest IOMMU is empty of
> > -         * mappings at this point.
> > -         *
> > -         * One way of doing this is:
> > -         * 1. Avoid sharing IOMMUs between emulated devices or different
> > -         * IOMMU groups.
> > -         * 2. Implement VFIO_IOMMU_ENABLE in the host kernel to fail if
> > -         * there are some mappings in IOMMU.
> > -         *
> > -         * VFIO on SPAPR does that. Other IOMMU models may do that 
> > different,
> > -         * they must make sure there are no existing mappings or
> > -         * loop through existing mappings to map them into VFIO.
> > -         */
> >          giommu = g_malloc0(sizeof(*giommu));
> >          giommu->iommu = section->mr;
> >          giommu->container = container;
> >          giommu->n.notify = vfio_iommu_map_notify;
> >          QLIST_INSERT_HEAD(&container->giommu_list, giommu, giommu_next);
> > +
> >          memory_region_register_iommu_notifier(giommu->iommu, &giommu->n);
> > +        memory_region_iommu_replay(giommu->iommu, &giommu->n,
> > +                                   vfio_container_granularity(container),
> > +                                   false);
> 
> I'm wondering if it has any sense to provide the "is_write" information
> at this level of the API: I don't think we can have access to this
> information when we call this function (so it will be always used with
> false, or called twice once with false, once with true). I think it
> would be better to manage this internally.

I agree it's pretty ugly, but I'm not really sure how to handle it
better.  The translate function itself wants is_write; I'm pretty sure
"false" is the right thing here, but I'm not sure it would be right
for all potential replay cases.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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