Re: [PATCH] [ppc64] watch IOMMU virtual merging

[Anton Blanchard <anton@samba.org>]

 
> This is a rather painful state of affairs. I'm not convinced external
> fragmentation in the IOMMU address space is insurmountable as the
> physically contiguous segments can (in principle; the mechanics of
> ramming this through the IO subsystem are another matter entirely) be
> IO-mapped in a bus-discontiguous fashion.

Yep they can, but if the SG list has been sized to fit after physically
merging then we have a problem when we later on want to split it. If the
architecture had more control over the merging process we could do a
better job here.

> I'm not familiar with the TCE space regions; could you describe or
> point to documentation for the semantics there?

Think of it as a mapping from PCI to host memory, a window of at least
128MB and up to 3GB. You get to carve it up on a page granularity as you
like, we currently create 2 regions, a small allocation region and a
large one (above 15 pages).  The aim is to (hopefully) avoid
fragmentation in the large allocation region.

> My first thought is to artificially limit the amount of physical
> merging (hopefully to some nonzero amount instead of disabling it
> entirely) allowed to take place in order to allow for better virtual
> merging.

Removing the large allocation region should also fix our problems. I
should test that to verify its our problem.

Anton