Re: [PATCH] intel-iommu: Document iova_tree

[Jason Wang <jasowang@redhat.com>]

On Fri, Dec 2, 2022 at 12:25 AM Peter Xu <peterx@redhat.com> wrote:
>
> It seems not super clear on when iova_tree is used, and why.  Add a rich
> comment above iova_tree to track why we needed the iova_tree, and when we
> need it.
>
> Suggested-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/hw/i386/intel_iommu.h | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/include/hw/i386/intel_iommu.h b/include/hw/i386/intel_iommu.h
> index 46d973e629..8d130ab2e3 100644
> --- a/include/hw/i386/intel_iommu.h
> +++ b/include/hw/i386/intel_iommu.h
> @@ -109,7 +109,35 @@ struct VTDAddressSpace {
>      QLIST_ENTRY(VTDAddressSpace) next;
>      /* Superset of notifier flags that this address space has */
>      IOMMUNotifierFlag notifier_flags;
> -    IOVATree *iova_tree;          /* Traces mapped IOVA ranges */
> +    /*
> +     * @iova_tree traces mapped IOVA ranges.
> +     *
> +     * The tree is not needed if no MAP notifiers is registered with
> +     * current VTD address space, because all UNMAP (including iotlb or
> +     * dev-iotlb) events can be transparently delivered to !MAP iommu
> +     * notifiers.

So this means the UNMAP notifier doesn't need to be as accurate as
MAP. (Should we document it in the notifier headers)?

For MAP[a, b] MAP[b, c] we can do a UNMAP[a. c].

> +     *
> +     * The tree OTOH is required for MAP typed iommu notifiers for a few
> +     * reasons.
> +     *
> +     * Firstly, there's no way to identify whether an PSI event is MAP or
> +     * UNMAP within the PSI message itself.  Without having prior knowledge
> +     * of existing state vIOMMU doesn't know whether it should notify MAP
> +     * or UNMAP for a PSI message it received.
> +     *
> +     * Secondly, PSI received from guest driver (or even a large PSI can
> +     * grow into a DSI at least with Linux intel-iommu driver) can be
> +     * larger in range than the newly mapped ranges for either MAP or UNMAP
> +     * events.

Yes, so I think we need a document that the UNMAP handler should be
prepared for this.

Thanks

> If it directly pass-throughs any such event it may confuse
> +     * the registered drivers (e.g. vfio-pci) on either: (1) trying to map
> +     * the same region more than once (for VFIO_IOMMU_MAP_DMA, -EEXIST will
> +     * trigger), or (2) trying to UNMAP a range that is still partially
> +     * mapped.  That accuracy is not required for UNMAP-only notifiers, but
> +     * it is a must-to-have for MAP-inclusive notifiers, because the vIOMMU
> +     * needs to make sure the shadow page table is always in sync with the
> +     * guest IOMMU pgtables for a device.
> +     */
> +    IOVATree *iova_tree;
>  };
>
>  struct VTDIOTLBEntry {
> --
> 2.37.3
>