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

On 2020/5/27 11:27, Tian, Kevin wrote:

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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:

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Hi,

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

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Hi all,

Is there any plan for enabling SMMU HTTU?

Not outside of SVA, as far as I know.

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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/

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It doesn't implement an internal API to communicate with the IOMMU

driver

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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. :)

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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.

By the way, I'm not sure whether HTTU or SLAD can help for mediated deivce.

-- 
Thanks,
Xiang


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