[RFC] Use SMMU HTTU for DMA dirty page tracking

[Jean-Philippe Brucker]

On Wed, May 27, 2020 at 08:40:47AM +0000, Tian, Kevin wrote:
> > From: Xiang Zheng <zhengxiang9 at huawei.com>
> > Sent: Wednesday, May 27, 2020 2:45 PM
> > 
> > 
> > On 2020/5/27 11:27, Tian, Kevin wrote:
> > >> 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 at 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).
> > >
> > 
> > Thank you, Kevin.
> > 
> > When will you send this series patches? Maybe(Hope) we can also support
> > hardware-based dirty pages tracking via common APIs based on your
> > patches. :)
> 
> Yan is working with Kirti on basic live migration support now. After that
> part is done, we will start working on A/D bit support. Yes, common APIs
> are definitely the goal here.
> 
> > 
> > >>
> > >>>
> > >>>> 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.
> > >
> > 
> > Currently HTTU just implement A/D-like mechanism in SMMU page tables.
> > We certainly
> > expect SMMU can also implement PML-like feature so that we can avoid
> > walking the
> > whole page table to get the dirty pages.

There is no reporting of dirty pages in log buffer. It might be possible
to do software logging based on PRI or Stall, but that requires special
support in the endpoint as well as the SMMU.

> Is there a link to HTTU introduction?

I don't know any gentle introduction, but there are sections D5.4.11
"Hardware management of the Access flag and dirty state" in the ARM
Architecture Reference Manual (DDI0487E), and section 3.13 "Translation
table entries and Access/Dirty flags" in the SMMU specification
(IHI0070C). HTTU stands for "Hardware Translation Table Update".

In short, when HTTU is enabled, the SMMU translation performs an atomic
read-modify-write on the leaf translation table descriptor, setting some
bits depending on the type of memory access. This can be enabled
independently on both stage-1 and stage-2 tables (equivalent to your 1st
and 2nd page tables levels, I think).

Thanks,
Jean