Hi Joerg,
On 31/03/15 15:24, Joerg Roedel wrote:
Hi Will,
On Fri, Mar 27, 2015 at 05:19:46PM +0000, Will Deacon wrote:
Please can you pull the following IOMMU changes for 4.1? They move the
per-iommu_ops pgsize_bitmap field into the iommu_domain, which allows
IOMMUs such as the ARM SMMU to support different page sizes within a
given SoC.
I have some concerns about the direction taken with this patch-set. The
goal for the IOMMU-API is still to have domains that can be attached to
arbitrary devices (even when mappings already exist). But with this
patch-set we move into a direction where a domain can only be used on
IOMMUs that support the page-sizes required by the domain. In the end
this would be visible to the user of the IOMMU-API, which is not what we
want.
I think the notion of sharing domains between arbitrary devices is
already broken for systems with multiple IOMMU devices - with the
iommu_domain itself now embedded in a private data structure, it's
certainly not safe to pass it to two different drivers, should the
client devices be behind heterogeneous IOMMUs. Furthermore, since most
of those private data structures contain instance-specific data, that
prevents a domain spanning even two homogeneous IOMMU devices.
If the goal is for iommu_domains to be a hardware-independent
abstraction, then shouldn't we be looking to move the other
instance-specific features like attributes and geometry out of them?
Perhaps the answer is the opposite; accept the existing iommu_domain as
what it appears to be used as - a fairly thin abstraction of one or more
context banks/uTLBs/DMARs/whatever on a single IOMMU device - and
consider some higher-level interface for grouping one or more of these
"device domains" together and sharing a single IOVA space between them.
Robin.
I can understand the motivation behind these patches, but we need to
think about how this could work with the desired semantics of the
IOMMU-API.
Joerg
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