Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management

[ebiederm@xmission.com (Eric W. Biederman)]

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.

Eric