Messages in this thread | | | From | "Yang, Shunyong" <> | Subject | Re: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict mode | Date | Wed, 15 Aug 2018 01:43:37 +0000 |
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Hi, Robin,
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?
Thanks. Shunyong.
> > Robin. | |