linux-kernel - Re: IOMMU keeping mapping over long period of time

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Date:	Thu, 20 Oct 2011 10:00:00 +0200
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org
Subject: Re: IOMMU keeping mapping over long period of time

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.


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