[RFC] Use SMMU HTTU for DMA dirty page tracking
prime.zeng at hisilicon.com
Wed May 27 06:46:24 UTC 2020
> -----Original Message-----
> From: zhengxiang (A)
> Sent: Monday, May 25, 2020 7:34 PM
> To: Jean-Philippe Brucker
> Cc: linux-arm-kernel at lists.infradead.org; kvmarm at lists.cs.columbia.edu;
> Will Deacon; Wanghaibin (D); Zengtao (B); maz at kernel.org; James Morse;
> julien.thierry.kdev at gmail.com; Suzuki K Poulose; Wangzhou (B);
> iommu at lists.linux-foundation.org; Kirti Wankhede; Yan Zhao;
> alex.williamson at redhat.com
> Subject: Re: [RFC] Use SMMU HTTU for DMA dirty page tracking
> [+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:
> de at nvidia.com/
> > It doesn't implement an internal API to communicate with the IOMMU
> > 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)
> > 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
> > would report all pages as they do now.
> > * VFIO_IOMMU_DIRTY_PAGES_FLAG_START/STOP would clear the dirty
> > 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?
I think we need START/STOP, start is called when migration is started and STOP
called when migration finished.
I think you mean that during the migration(between START and STOP), HA and HD
functionality is always turned on.
> > * VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP would query the
> dirty bit for all
> > VFIO mappings.
We need a clear during the query which mean we have to reset the dirty status
after a query.
> I think we need to consider the case of IOMMU dirty pages logging. We
> to test Kirti's VFIO migration patches combined with SMMU HTTU, any
@kirti @yan @alex
As we know, you have worked on this feature for a long time, and it seems that
everything is going very well now, thanks very much for your VFIO migration
framework, it really helps a lot, and we want to start with SMMU HTTU enabled
based on your jobs, it's kind of hardware dirty page tracking, and expected to bring us
better performance, any suggestions?
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