Re: [PATCH 0/4] Generic IOMMU page table framework

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

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