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From:   "Yang, Shunyong" <shunyong.yang@hxt-semitech.com>
To:     Will Deacon <will.deacon@arm.com>
CC:     "jean-philippe.brucker@arm.com" <jean-philippe.brucker@arm.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
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        "thunder.leizhen@huawei.com" <thunder.leizhen@huawei.com>,
        "robin.murphy@arm.com" <robin.murphy@arm.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict
 mode
Thread-Topic: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for
 non-strict mode
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Date:   Thu, 16 Aug 2018 00:43:45 +0000
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Hi, Will and Robin,

  Many thanks for your explanations.

Thanks.
Shunyong.

On 2018/8/15 15:35, Will Deacon wrote:
> On Wed, Aug 15, 2018 at 08:33:01AM +0100, Will Deacon wrote:
>> On Wed, Aug 15, 2018 at 01:43:37AM +0000, Yang, Shunyong wrote:
>>> On Tue, 2018-08-14 at 11:02 +0100, Robin Murphy wrote:
>>>> On 14/08/18 09:35, Will Deacon wrote:
>>>>> On Tue, Aug 14, 2018 at 04:33:41PM +0800, Leizhen (ThunderTown)
>>>>> wrote:
>>>>>> On 2018/8/6 9:32, Yang, Shunyong wrote:
>>>>>>> On 2018/7/26 22:37, Robin Murphy wrote:
>>>>>>>> Because DMA code is not the only caller of iommu_map/unmap.
>>>>>>>> It's
>>>>>>>> perfectly legal in the IOMMU API to partially unmap a
>>>>>>>> previous mapping
>>>>>>>> such that a block entry needs to be split. The DMA API,
>>>>>>>> however, is a
>>>>>>>> lot more constrined, and thus by construction the iommu-dma
>>>>>>>> layer will
>>>>>>>> never generate a block-splitting iommu_unmap() except as a
>>>>>>>> result of
>>>>>>>> illegal DMA API usage, and we obviously do not need to
>>>>>>>> optimise for that
>>>>>>>> (you will get a warning about mismatched unmaps under dma-
>>>>>>>> debug, but
>>>>>>>> it's a bit too expensive to police in the general case).
>>>>>>>>
>>>>>>>
>>>>>>> When I was reading the code around arm_lpae_split_blk_unmap(),
>>>>>>> I was
>>>>>>> curious in which scenario a block will be split. Now with your
>>>>>>> comments
>>>>>>> "Because DMA code is not the only caller of iommu_map/unmap",
>>>>>>> it seems
>>>>>>> depending on the user.
>>>>>>>
>>>>>>> Would you please explain this further? I mean besides DMA,
>>>>>>> which user
>>>>>>> will use iommu_map/umap and how it split a block.
>>>>>>
>>>>>> I also think that arm_lpae_split_blk_unmap() scenario is not
>>>>>> exist, maybe
>>>>>> we should remove it, and give a warning for this wrong usage.
>>>>>
>>>>> Can't it happen with VFIO?
>>>>
>>>> ...or GPU drivers, or anyone else managing their own IOMMU domain
>>>> directly. A sequence like this is perfectly legal:
>>>>
>>>>     iommu_map(domain, iova, paddr, SZ_8M, prot);
>>>>     ...
>>>>     iommu_unmap(domain, iova + SZ_1M * 5, SZ_1M * 3);
>>>>
>>>> where if iova and paddr happen to be suitably aligned, the map will
>>>> lay
>>>> down blocks, and the unmap will then have to split one of them into
>>>> pages to remove half of it. We don't tear our hair out maintaining
>>>> split_blk_unmap() for the fun of it :(
>>>
>>> Thank you for the GPU example. But for VFIO, I remember all memory will
>>> be   pinned in the early stage of emulator (such as qemu) start. So,
>>> the split will occur at which operation? Maybe virtio balloon inflate?
>>
>> My memory is pretty hazy here, but I was fairly sure that VFIO didn't
>> always unmap() with the same granularity as it map()'d, at least for
>> the v1 interface. Either way, split_blk_unmap() was written because it was
>> necessary at the time, rather than just for fun!
>>
>> Will
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> 
> Urgh, sorry about this threatening disclaimer ^^. Please disregard.
> 
> Will
>