Generic IOMMU page table framework

Quoted 22 lines · Laurent Pinchart

Quoted 21 lines · 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.

Quoted 7 lines

Quoted 4 lines

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.