linux-kernel - Re: [PATCH v2 04/11] iommu/arm-smmu-v3: Split block descriptor when start dirty log

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Date:   Tue, 16 Mar 2021 19:39:47 +0800
From:   Keqian Zhu <zhukeqian1@...wei.com>
To:     Yi Sun <yi.y.sun@...ux.intel.com>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        "Will Deacon" <will@...nel.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "Cornelia Huck" <cohuck@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        <wanghaibin.wang@...wei.com>, <jiangkunkun@...wei.com>,
        <yuzenghui@...wei.com>, <lushenming@...wei.com>
Subject: Re: [PATCH v2 04/11] iommu/arm-smmu-v3: Split block descriptor when
 start dirty log

Hi Yi,

On 2021/3/16 17:17, Yi Sun wrote:
> On 21-03-10 17:06:07, Keqian Zhu wrote:
>> From: jiangkunkun <jiangkunkun@...wei.com>
>>
>> Block descriptor is not a proper granule for dirty log tracking.
>> Take an extreme example, if DMA writes one byte, under 1G mapping,
>> the dirty amount reported to userspace is 1G, but under 4K mapping,
>> the dirty amount is just 4K.
>>
>> This adds a new interface named start_dirty_log in iommu layer and
>> arm smmuv3 implements it, which splits block descriptor to an span
>> of page descriptors. Other types of IOMMU will perform architecture
>> specific actions to start dirty log.
>>
>> To allow code reuse, the split_block operation is realized as an
>> iommu_ops too. We flush all iotlbs after the whole procedure is
>> completed to ease the pressure of iommu, as we will hanle a huge
>> range of mapping in general.
>>
>> Spliting block does not simultaneously work with other pgtable ops,
>> as the only designed user is vfio, which always hold a lock, so race
>> condition is not considered in the pgtable ops.
>>
>> Co-developed-by: Keqian Zhu <zhukeqian1@...wei.com>
>> Signed-off-by: Kunkun Jiang <jiangkunkun@...wei.com>
>> ---
>>
>> changelog:
>>
>> v2:
>>  - Change the return type of split_block(). size_t -> int.
>>  - Change commit message to properly describe race condition. (Robin)
>>  - Change commit message to properly describe the need of split block.
>>  - Add a new interface named start_dirty_log(). (Sun Yi)
>>  - Change commit message to explain the realtionship of split_block() and start_dirty_log().
>>
>> ---
>>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c |  52 +++++++++
>>  drivers/iommu/io-pgtable-arm.c              | 122 ++++++++++++++++++++
>>  drivers/iommu/iommu.c                       |  48 ++++++++
>>  include/linux/io-pgtable.h                  |   2 +
>>  include/linux/iommu.h                       |  24 ++++
>>  5 files changed, 248 insertions(+)
>>
> Could you please split iommu common interface to a separate patch?
> This may make review and comments easier.
Yup, good suggestion.

> 
> IMHO, I think the start/stop interfaces could be merged into one, e.g:
>     int iommu_domain_set_hwdbm(struct iommu_domain *domain, bool enable,
>                                unsigned long iova, size_t size,
>                                int prot);
Looks good, this reduces some code. but I have a concern that this causes loss of flexibility,
as we must pass same arguments when start|stop dirty log. What's your opinion about this?

> 
> Same comments to patch 5.
OK. Thanks.

> 
> BRs,
> Yi Sun
> 
>> -- 
>> 2.19.1
> .
Thanks,
Keqian