exec: add page_mask for flatview_do_translate

exec: add page_mask for flatview_do_translate

The function is originally used for flatview_space_translate() and what
we care about most is (xlat, plen) range. However for iotlb requests, we
don't really care about "plen", but the size of the page that "xlat" is
located on. While, plen cannot really contain this information.

A simple example to show why "plen" is not good for IOTLB translations:

E.g., for huge pages, it is possible that guest mapped 1G huge page on
device side that used this GPA range:

  0x100000000 - 0x13fffffff

Then let's say we want to translate one IOVA that finally mapped to GPA
0x13ffffe00 (which is located on this 1G huge page). Then here we'll
get:

  (xlat, plen) = (0x13fffe00, 0x200)

So the IOTLB would be only covering a very small range since from
"plen" (which is 0x200 bytes) we cannot tell the size of the page.

Actually we can really know that this is a huge page - we just throw the
information away in flatview_do_translate().

This patch introduced "page_mask" optional parameter to capture that
page mask info. Also, I made "plen" an optional parameter as well, with
some comments for the whole function.

No functional change yet.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Message-Id: <20171010094247.10173-2-maxime.coquelin@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>