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

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Xiang Zheng
> Sent: Monday, May 25, 2020 7:34 PM
> 
> [+cc Kirti, Yan, Alex]
> 
> On 2020/5/23 1:14, Jean-Philippe Brucker wrote:
> > 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/
> > It doesn't implement an internal API to communicate with the IOMMU
> driver
> > about dirty pages.

We plan to add such API later, e.g. to utilize A/D bit in VT-d 2nd-level 
page tables (Rev 3.0). 

> 
> >
> >> 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.

Does HTTU implement A/D-like mechanism in SMMU page tables, or just
report dirty pages in a log buffer? Either way tracking dirty pages in IOMMU
side is generic thus doesn't require device-specific tweak like in Intel NIC.

Thanks
kevin

> >
> > 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.
> 
> Maybe we need not switch HA and HD bits, just turn on them all the time?
> 
> >
> > * VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP would query the dirty
> bit for all
> >   VFIO mappings.
> >
> 
> I think we need to consider the case of IOMMU dirty pages logging. We want
> to test Kirti's VFIO migration patches combined with SMMU HTTU, any
> suggestions?
> 
> --
> Thanks,
> Xiang
> 
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