On Sun, Nov 30, 2014 at 10:03:08PM +0000, Laurent Pinchart wrote: > Hi Will,
Hi Laurent, > On Thursday 27 November 2014 11:51:14 Will Deacon wrote: > > Hi all, > > > > This series introduces a generic IOMMU page table allocation framework, > > implements support for ARM long-descriptors and then ports the arm-smmu > > driver over to the new code. > > > > There are a few reasons for doing this: > > > > - Page table code is hard, and I don't enjoy shopping > > > > - A number of IOMMUs actually use the same table format, but currently > > duplicate the code > > > > - It provides a CPU (and architecture) independent allocator, which > > may be useful for some systems where the CPU is using a different > > table format for its own mappings > > > > As illustrated in the final patch, an IOMMU driver interacts with the > > allocator by passing in a configuration structure describing the > > input and output address ranges, the supported pages sizes and a set of > > ops for performing various TLB invalidation and PTE flushing routines. > > > > The LPAE code implements support for 4k/2M/1G, 16k/32M and 64k/512M > > mappings, but I decided not to implement the contiguous bit in the > > interest of trying to keep the code semi-readable. This could always be > > added later, if needed. > > Do you have any idea how much the contiguous bit can improve performances in > real use cases ? It depends on the TLB, really. Given that the contiguous sized map directly onto block sizes using different granules, I didn't see that the complexity was worth it. For example: 4k granule : 16 contiguous entries => {64k, 32M, 16G} 16k granule : 128 contiguous lvl3 entries => 2M 32 contiguous lvl2 entries => 1G 64k granule : 32 contiguous entries => {2M, 16G} If we use block mappings, then we get: 4k granule : 2M @ lvl2, 1G @ lvl1 16k granule : 32M @ lvl2 64k granule : 512M @ lvl2 so really, we only miss the ability to create 16G mappings. I doubt that hardware even implements that size in the TLB (the contiguous bit is only a hint). On top of that, the contiguous bit leads to additional expense on unmap, since you have extra TLB invalidation splitting the thing into non-contiguous pages before you can do anything. Will _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu