Messages in this thread | | | From | "Yang, Shunyong" <> | Subject | Re: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict mode | 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 >> IMPORTANT NOTICE: The contents of this email and any attachments are >> confidential and may also be privileged. If you are not the intended >> recipient, please notify the sender immediately and do not disclose the >> contents to any other person, use it for any purpose, or store or copy the >> information in any medium. Thank you. > > Urgh, sorry about this threatening disclaimer ^^. Please disregard. > > Will >
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