Re: IOMMU keeping mapping over long period of time

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2011-10-18 at 11:48 -0400, Jerome Glisse wrote:
> Hi,
> 
> For GPU we are entertaining the idea of keeping pci/dma mapping of page
> around for the whole life span of GPU object (which can be as long as
> the computer uptime). Here we are talking about possibly several GB of
> data.
> 
> GPU for which we are interested have dma mask of 40bits at least, thus
> they can work with high mem page. My main concern is when an iommu is
> active, such amount of continuously mapped page might exhaust the iommu
> address space. My understanding is that Intel & AMD iommu are capable of
> growing their address space but is it somethings we can rely on ?

This will be problematic on other platforms. Also keep in mind that
objects mapped in the iommu can no longer be swapped out.

It might be useful to have a more 'lazy' mechanism where objects are
kept mapped but with a callback to unmap them in case of iommu pressure.

The problem of course with such a scheme is that it requires iommu
object tracking and horrible locking issues.

Ben.