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From:   Kunkun Jiang <jiangkunkun@huawei.com>
Subject: [question] Assign multiple devices from different SMMUs to a
 arm_smmu_domain
To:     Robin Murphy <robin.murphy@arm.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>
Message-ID: <bbfcb875-0da1-c303-ed48-fdaa890c89b3@huawei.com>
Date:   Tue, 7 Sep 2021 15:41:52 +0800
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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.

Looking forward to your suggestions.????

Thanks,
Kunkun Jiang