Re: [RFC 0/6] iommu: generic api migration and grouping

[Roedel, Joerg]

On Mon, Jun 06, 2011 at 12:36:13PM -0400, Ohad Ben-Cohen wrote:
> On Mon, Jun 6, 2011 at 6:35 PM, Roedel, Joerg <Joerg.Roedel@xxxxxxx> wrote:
> > On Mon, Jun 06, 2011 at 11:15:30AM -0400, Ohad Ben-Cohen wrote:
> >
> >> This is insufficient; users need somehow to tell what page sizes are
> >> supported by the underlying hardware (we can't assume host page-sizes,
> >> and we want to use bigger pages whenever possible, to relax the TLB
> >> pressure).
> > /
> > What does the IOMMU-API user need this info for? On the x86 IOMMUs these
> > details are handled transparently by the IOMMU driver.
> 
> That's one way to do that, but then it means duplicating this logic
> inside the different IOMMU implementations.
> 
> Take the OMAP (and seemingly MSM too) example: we have 4KB, 64KB, 1MB
> and 16MB page-table entries. When we map a memory region, we need to
> break it up to a minimum number of pages (while validating
> sizes/alignments are sane). It's not complicated, but it can be nice
> if it'd be implemented only once.

Well, it certainly makes sense to have a single implementation for this.
But I want to hide this complexity to the user of the IOMMU-API. The
best choice is to put this into the layer between the IOMMU-API and the
backend implementation.

> In addition, unless we require 'va' and 'pa' to have the exact same
> alignment, we might run into specific page configuration that the
> IOMMU implementation cannot restore on ->unmap, since unmap only takes
> 'va' and 'order'. So we will either have to supply 'pa' too, or have
> the implementation remember the mapping in order to unmap it later.
> That begins to be a bit messy...

That interface is not put into stone. There were other complains about
the ->unmap part recently, so there is certainly room for improvement
there.

Regards,

	Joerg

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