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Subject: Re: [question] Assign multiple devices from different SMMUs to a
 arm_smmu_domain
To:     Kunkun Jiang <jiangkunkun@huawei.com>,
        Will Deacon <will@kernel.org>,
        Linux IOMMU <iommu@lists.linux-foundation.org>,
        open list <linux-kernel@vger.kernel.org>
Cc:     wanghaibin.wang@huawei.com, Zenghui Yu <yuzenghui@huawei.com>,
        Keqian Zhu <zhukeqian1@huawei.com>
References: <bbfcb875-0da1-c303-ed48-fdaa890c89b3@huawei.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <202f32cd-8036-563e-028b-781b999766be@arm.com>
Date:   Tue, 7 Sep 2021 10:43:05 +0100
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On 2021-09-07 08:41, Kunkun Jiang wrote:
> Hi all,
> 
> I am working on VFIO DMA dirty pages tracking based on ARM SMMU HTTU,
> and have done a lot of testing.In the test, I found a problem that 
> greatly affects
> performance of VFIO DMA dirty pages tracking.
> 
> According to the current arm-smmu-v3 driver, multiple VFIO pass-through
> device comes from different SMMUs will be assigned to different
> arm_smmu_domain. It will create page table for each arm_smmu_domain,
> even though these page tables are exactly the same. Bacause dirty pages
> tracking needs to traverse the page table, multiple page tables will make
> performance worse.
> 
> I learned the ARM SMMUv3 spec and had some exchanges with my colleagues
> who work on SMMU hardware. I did not find the restriction that multiple
> SMMUs cannot share the same page table. We migth be able to do this like
> x86 IOMMU. If I have missed something, please point it out.

Sure, it's not impossible, there are just a lot of fiddly 
considerations, mostly around how to handle SMMU instances with 
different capabilities. We haven't had a strong need to support this 
case so far, so we've simply been avoiding all that complexity.

Robin.

> Looking forward to your suggestions.????
> 
> Thanks,
> Kunkun Jiang
>