How to allocate contiguous RAM in pv guests

[Uli <um@amd64.org>]

Hi!

I'm working on a patch to get the GART as an IOMMU working in linux/dom0.
However, the problem I describe below applies equally to the software
IOMMU.

If the BIOS doesn't setup the aperture it has to be allocated from
memory. Therefore, one needs a contiguous memory region, currently 64MB.
The software IOMMU always needs a contiguous memory region (same size).

In order to do this, the hypercall XENMEM_exchange is given a bunch of
mfns and returns a (host physically) contiguous memory region.
Unfortunately, the implementation allocates the contiguous memory from
the heap first and then returns the discontiguous mfns to it. Therefore,
there has to be a (in this case) 64MB chunk of memory in the xen heap
available for the call to succeed.
I have observed that on most machines exactly one such chunk is
available. However, I've also had a machine where this is not the case.

It seems to me that using the xen heap is not the right thing to do.
The only other option I can think of is scanning dom0's memory for a
(host physical) chunk of memory that
a) belongs entirely to it and
b) is free

Once such a chunk is found, one would have to map it contiguously into
virtual memory.  Actually, the latter is only neccessary for the
software IOMMU. The GART aperture doesn't have to be in virtual memory
since it is only accessed from devices.

Thanks for any suggestions,

Uli