Re: [RFC 3/3] mm: iommu: The Virtual Contiguous Memory Manager

[Zach Pfeffer]

Daniel Walker wrote:
> On Thu, 2010-07-01 at 15:00 -0700, Zach Pfeffer wrote:
> 
> 
>> Additionally, the current IOMMU interface does not allow users to
>> associate one page table with multiple IOMMUs unless the user explicitly
>> wrote a muxed device underneith the IOMMU interface. This also could be
>> done, but would have to be done for every such use case. Since the
>> particular topology is run-time configurable all of these use-cases and
>> more can be expressed without pushing the topology into the low-level
>> IOMMU driver.
>>
>> The VCMM takes the long view. Its designed for a future in which the
>> number of IOMMUs will go up and the ways in which these IOMMUs are
>> composed will vary from system to system, and may vary at
>> runtime. Already, there are ~20 different IOMMU map implementations in
>> the kernel. Had the Linux kernel had the VCMM, many of those
>> implementations could have leveraged the mapping and topology management
>> of a VCMM, while focusing on a few key hardware specific functions (map
>> this physical address, program the page table base register).
> 
> So if we include this code which "map implementations" could you
> collapse into this implementations ? Generally , what currently existing
> code can VCMM help to eliminate?

In theory, it can eliminate all code the interoperates between IOMMU,
CPU and non-IOMMU based devices and all the mapping code, alignment,
mapping attribute and special block size support that's been
implemented.


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