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

[Laurent Vivier <lvivier@redhat.com>]

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 <david@gibson.dropbear.id.au>
> ---
>  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.

-
>  
>          return;
>      }
>