GNTTABOP_unmap_grant_ref doc

[David Scott <dave.scott@eu.citrix.com>]

Hi,

I've been working on the Mirage (mini-OS + OCaml runtime) blkback 
implementation. It seems to be working much more happily now but I was a 
bit surprised by the GNTTABOP_unmap_grant_ref doc -- maybe I misread it.

In ./xen/include/public/grant_table.h it says

/*
  * GNTTABOP_unmap_grant_ref: Destroy one or more grant-reference mappings
  * tracked by <handle>. If <host_addr> or <dev_bus_addr> is zero, that
  * field is ignored. If non-zero, they must refer to a device/host mapping
  * that is tracked by <handle>
  * NOTES:
  *  1. The call may fail in an undefined manner if either mapping is not
  *     tracked by <handle>.
  *  3. After executing a batch of unmaps, it is guaranteed that no stale
  *     mappings will remain in the device or host TLBs.
  */

When I read
   "If <host_addr> or <dev_bus_addr> is zero, that field is ignored"
   "The call may fail in an undefined manner if either mapping is not 
tracked by <handle>"

I thought, "great, I'll not bother tracking anything except the handle 
and set both fields to zero". However if I set host_addr to zero then my 
frontend complains bitterly

gnttab_stubs.c: WARNING: g.e. 53 still in use! (19)

If I store the host_addr and fill it in during the unmap, everything 
seems to work.

So did I misread the doc? Or did it mean exclusive-or i.e. you must fill 
in either one of these two fields? Or have I done something stupid 
elsewhere (always possible)?

If it turns out that I have misread the doc, I'll happily send a patch 
to improve the text (once I'm sure I understand what it should say...)

Thanks!
Dave