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    SubjectRe: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management
    On Wed, Jul 14, 2010 at 06:47:34PM -0700, Eric W. Biederman wrote:
    > Zach Pfeffer <zpfeffer@codeaurora.org> writes:
    >
    > > On Wed, Jul 14, 2010 at 11:05:36PM +0100, Russell King - ARM Linux wrote:
    > >> On Wed, Jul 14, 2010 at 01:11:49PM -0700, Zach Pfeffer wrote:
    > >> > If the DMA-API contained functions to allocate virtual space separate
    > >> > from physical space and reworked how chained buffers functioned it
    > >> > would probably work - but then things start to look like the VCM API
    > >> > which does graph based map management.
    > >>
    > >> Every additional virtual mapping of a physical buffer results in
    > >> additional cache aliases on aliasing caches, and more workload for
    > >> developers to sort out the cache aliasing issues.
    > >>
    > >> What does VCM to do mitigate that?
    > >
    > > The VCM ensures that all mappings that map a given physical buffer:
    > > IOMMU mappings, CPU mappings and one-to-one device mappings all map
    > > that buffer using the same (or compatible) attributes. At this point
    > > the only attribute that users can pass is CACHED. In the absence of
    > > CACHED all accesses go straight through to the physical memory.
    > >
    > > The architecture of the VCM allows these sorts of consistency checks
    > > to be made since all mappers of a given physical resource are
    > > tracked. This is feasible because the physical resources we're
    > > tracking are typically large.
    >
    > On x86 this is implemented in the pat code, and could reasonably be
    > generalized to be cross platform.
    >
    > This is controlled by HAVE_PFNMAP_TRACKING and with entry points
    > like track_pfn_vma_new.
    >
    > Given that we already have an implementation that tracks the cached
    > vs non-cached attribute using the dma api. I don't see that the
    > API has to change. An implementation of the cached vs non-cached
    > status for arm and other architectures is probably appropriate.
    >
    > It is definitely true that getting your mapping caching attributes
    > out of sync can be a problem.

    Sure, but we're still stuck with needing lots of scatterlist list
    elements and needing to copy them to share physical buffers.


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