Re: [RFC] Use SMMU HTTU for DMA dirty page tracking

Hi,

On Tue, May 19, 2020 at 05:42:55PM +0800, Xiang Zheng wrote:

Hi all,

Is there any plan for enabling SMMU HTTU?

Not outside of SVA, as far as I know.

I have seen the patch locates in the SVA series patch, which adds
support for HTTU:
    https://www.spinics.net/lists/arm-kernel/msg798694.html

HTTU reduces the number of access faults on SMMU fault queue
(permission faults also benifit from it).

Besides reducing the faults, HTTU also helps to track dirty pages for
device DMA. Is it feasible to utilize HTTU to get dirty pages on device
DMA during VFIO live migration?

As you know there is a VFIO interface for this under discussion:
https://lore.kernel.org/kvm/1589781397-28368-1-git-send-email-kwankhede@nvidia.com/ (local)
It doesn't implement an internal API to communicate with the IOMMU driver
about dirty pages.

If SMMU can track dirty pages, devices are not required to implement
additional dirty pages tracking to support VFIO live migration.

It seems feasible, though tracking it in the device might be more
efficient. I might have misunderstood but I think for live migration of
the Intel NIC they trap guest accesses to the device and introspect its
state to figure out which pages it is accessing.

With HTTU I suppose (without much knowledge about live migration) that
you'd need several new interfaces to the IOMMU drivers:

* A way for VFIO to query HTTU support in the SMMU. There are some
  discussions about communicating more IOMMU capabilities through VFIO but
  no implementation yet. When HTTU isn't supported the DIRTY_PAGES bitmap
  would report all pages as they do now.

* VFIO_IOMMU_DIRTY_PAGES_FLAG_START/STOP would clear the dirty bit
  for all VFIO mappings (which is going to take some time). There is a
  walker in io-pgtable for iova_to_phys() which could be extended. I
  suppose it's also possible to atomically switch the HA and HD bits in
  context descriptors.

* VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP would query the dirty bit for all
  VFIO mappings.

Thanks,
Jean

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