Re: [Qemu-devel] [PATCHv3 5/7] memory: Allow replay of IOMMU mapping notifications

[Laurent Vivier <lvivier@redhat.com>]

On 30/09/2015 04:13, David Gibson wrote:
> When we have guest visible IOMMUs, we allow notifiers to be registered
> which will be informed of all changes to IOMMU mappings.  This is used by
> vfio to keep the host IOMMU mappings in sync with guest IOMMU mappings.
> 
> However, unlike with a memory region listener, an iommu notifier won't be
> told about any mappings which already exist in the (guest) IOMMU at the
> time it is registered.  This can cause problems if hotplugging a VFIO
> device onto a guest bus which had existing guest IOMMU mappings, but didn't
> previously have an VFIO devices (and hence no host IOMMU mappings).
> 
> This adds a memory_region_iommu_replay() function to handle this case.  It
> replays any existing mappings in an IOMMU memory region to a specified
> notifier.  Because the IOMMU memory region doesn't internally remember the
> granularity of the guest IOMMU it has a small hack where the caller must
> specify a granularity at which to replay mappings.
> 
> If there are finer mappings in the guest IOMMU these will be reported in
> the iotlb structures passed to the notifier which it must handle (probably
> causing it to flag an error).  This isn't new - the VFIO iommu notifier
> must already handle notifications about guest IOMMU mappings too short
> for it to represent in the host IOMMU.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  include/exec/memory.h | 13 +++++++++++++
>  memory.c              | 20 ++++++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 5baaf48..0f07159 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -583,6 +583,19 @@ void memory_region_notify_iommu(MemoryRegion *mr,
>  void memory_region_register_iommu_notifier(MemoryRegion *mr, Notifier *n);
>  
>  /**
> + * memory_region_iommu_replay: replay existing IOMMU translations to
> + * a notifier
> + *
> + * @mr: the memory region to observe
> + * @n: the notifier to which to replay iommu mappings
> + * @granularity: Minimum page granularity to replay notifications for
> + * @is_write: Whether to treat the replay as a translate "write"
> + *     through the iommu
> + */
> +void memory_region_iommu_replay(MemoryRegion *mr, Notifier *n,
> +                                hwaddr granularity, bool is_write);
> +
> +/**
>   * memory_region_unregister_iommu_notifier: unregister a notifier for
>   * changes to IOMMU translation entries.
>   *
> diff --git a/memory.c b/memory.c
> index ef87363..1b03d22 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -1403,6 +1403,26 @@ void memory_region_register_iommu_notifier(MemoryRegion *mr, Notifier *n)
>      notifier_list_add(&mr->iommu_notify, n);
>  }
>  
> +void memory_region_iommu_replay(MemoryRegion *mr, Notifier *n,
> +                                hwaddr granularity, bool is_write)
> +{
> +    hwaddr addr;
> +    IOMMUTLBEntry iotlb;
> +
> +    for (addr = 0; addr < memory_region_size(mr); addr += granularity) {
> +        iotlb = mr->iommu_ops->translate(mr, addr, is_write);

in iotlb, there is an "address_mask", on spapr, it is copied from
"page_shift", which is SPAPR_TCE_PAGE_SHIFT (12 -> 4k).

At a first glance, we would like to use it to scan the memory region,
but as granularity could be a greater value, I think it is a better choice.

But the question is: why the iotlb page_size is not equal to the
granularity given by VFIO_IOMMU_GET_INFO _IO ?

> +        if (iotlb.perm != IOMMU_NONE) {
> +            n->notify(n, &iotlb);
> +        }
> +
> +        /* if (2^64 - MR size) < granularity, it's possible to get an
> +         * infinite loop here.  This should catch such a wraparound */
> +        if ((addr + granularity) < addr) {
> +            break;
> +        }
> +    }
> +}
> +
>  void memory_region_unregister_iommu_notifier(Notifier *n)
>  {
>      notifier_remove(n);
> 

As my question is not a bout this particular patch but on another
existing part, I can say:

Reviewed-by: Laurent Vivier <lvivier@redhat.com>