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

From: Xiang Zheng <zhengxiang9@huawei.com>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: alex.williamson@redhat.com, Yan Zhao <yan.y.zhao@intel.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	maz@kernel.org, iommu@lists.linux-foundation.org,
	Kirti Wankhede <kwankhede@nvidia.com>,
	James Morse <james.morse@arm.com>,
	julien.thierry.kdev@gmail.com, prime.zeng@hisilicon.com,
	Wang Haibin <wanghaibin.wang@huawei.com>,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC] Use SMMU HTTU for DMA dirty page tracking
Date: Mon, 25 May 2020 19:34:18 +0800	[thread overview]
Message-ID: <e68c1158-8573-a477-42ce-48cee510c3ce@huawei.com> (raw)
In-Reply-To: <20200522171452.GC3453945@myrica>

[+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.

> 
>> 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.

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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