Re: [Qemu-devel] [RFC PATCH] vfio: VFIO Driver core framework

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

On Tue, 2011-11-15 at 11:05 +1100, David Gibson wrote:
> Being strict, or at least enforcing strictness, requires that the
> infrastructure track all the maps, so that the unmaps can be
> matching.  This is not a natural thing with the data structures you
> want for all IOMMUs.  For example on POWER, the IOMMU (aka TCE table)
> is a simple 1-level pagetable.  One pointer with a couple of
> permission bits per IOMMU page.  Handling oddly overlapping operations
> on that data structure is natural, enforcing strict matching of maps
> and unmaps is not and would require extra information to be stored by
> vfio.  On POWER, the IOMMU operations often *are* a hot path, so
> manipulating those structures would have a real cost, too. 

In fact they are a very hot path even. There's no way we can afford the
cost of tracking per page mapping/unmapping (other than bumping the page
count on a page that's currently mapped or via some debug-only feature).

Cheers,
Ben.