Sorry for the delayed answer, but I look at the vmap_pfn usage in the
previous version and tried to come up with a better version.  This
mostly untested branch:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/hyperv-vmap

get us there for swiotlb and the channel infrastructure  I've started
looking at the network driver and didn't get anywhere due to other work.

As far as I can tell the network driver does gigantic multi-megabyte
vmalloc allocation for the send and receive buffers, which are then
passed to the hardware, but always copied to/from when interacting
with the networking stack.  Did I see that right?  Are these big
buffers actually required unlike the normal buffer management schemes
in other Linux network drivers?

If so I suspect the best way to allocate them is by not using vmalloc
but just discontiguous pages, and then use kmap_local_pfn where the
PFN includes the share_gpa offset when actually copying from/to the
skbs.