When unmapping a buffer from an IOMMU domain, the IOMMU framework unmaps the buffer at a granule of the largest page size that is supported by the IOMMU hardware and fits within the buffer. For every block that is unmapped, the IOMMU framework will call into the IOMMU driver, and then the io-pgtable framework to walk the page tables to find the entry that corresponds to the IOVA, and then unmaps the entry.
This can be suboptimal in scenarios where a buffer or a piece of a buffer can be split into several contiguous page blocks of the same size. For example, consider an IOMMU that supports 4 KB page blocks, 2 MB page blocks, and 1 GB page blocks, and a buffer that is 4 MB in size is being unmapped at IOVA 0. The current call-flow will result in 4 indirect calls, and 2 page table walks, to unmap 2 entries that are next to each other in the page-tables, when both entries could have been unmapped in one shot by clearing both page table entries in the same call. These patches implement a callback called unmap_pages to the io-pgtable code and IOMMU drivers which unmaps an IOVA range that consists of a number of pages of the same page size that is supported by the IOMMU hardware, and allows for clearing multiple entries in the same set of indirect calls. The reason for introducing unmap_pages is to give other IOMMU drivers/io-pgtable formats time to change to using the new unmap_pages callback, so that the transition to using this approach can be done piecemeal. The same optimization is applicable for mapping buffers, however, the error handling in the io-pgtable layer couldn't be handled cleanly, as we would need to invoke iommu_unmap to unmap the parts of the buffer that were mapped, and then do any TLB maintenance. However, that seemed like a layering violation. Any feedback is very much appreciated. Thanks, Isaac Isaac J. Manjarres (5): iommu/io-pgtable: Introduce unmap_pages() as a page table op iommu: Add an unmap_pages() op for IOMMU drivers iommu: Add support for the unmap_pages IOMMU callback iommu/io-pgtable-arm: Implement arm_lpae_unmap_pages() iommu/arm-smmu: Implement the unmap_pages IOMMU driver callback drivers/iommu/arm/arm-smmu/arm-smmu.c | 19 +++++ drivers/iommu/io-pgtable-arm.c | 114 +++++++++++++++++++++----- drivers/iommu/iommu.c | 44 ++++++++-- include/linux/io-pgtable.h | 4 + include/linux/iommu.h | 4 + 5 files changed, 159 insertions(+), 26 deletions(-) -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
