libxl: ocaml: event management

Having bindings to the low-level functions libxl_osevent_register_hooks and
related, allows to run an event loop in OCaml; either one we write ourselves,
or one that is available elsewhere.

The Lwt cooperative threads library (http://ocsigen.org/lwt/), which is quite
popular these days, has an event loop that can be easily extended to poll any
additional fds that we get from libxl. Lwt provides a "lightweight" threading
model, which does not let you run any other (POSIX) threads in your
application, and therefore excludes an event loop implemented in the C
bindings.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
CC: David Scott <dave.scott@eu.citrix.com>
CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>

---
v6: Register "user" values with the OCaml GC, before handing them over to
libxl, and unregister when giving them back to OCaml (the latter applies only
to the for_callback values that go into asynchronous libxl calls; the (os)event
ones are registered once and kept forever).

I removed the acks I had on this patch, because of the relatively significant
changes.
libxl: ocaml: allow device operations to be called asynchronously

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
CC: David Scott <dave.scott@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>

---
v6: Updated patch to register for_callback value with the GC.
libxl: ocaml: add disk and cdrom helper functions

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
libxl: ocaml: add VM lifecycle operations

Also, reorganise toplevel OCaml functions into modules of Xenlight.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
CC: David Scott <dave.scott@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>

---
v6: Updated aohow_val to register for_callback with the OCaml GC.
libxl: ocaml: add console reader functions

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
libxl: ocaml: drop the ocaml heap lock before calling into libxl

Ocaml has a heap lock which must be held whenever ocaml code is running. Ocaml
usually drops this lock when it enters a potentially blocking low-level
function, such as writing to a file. Libxl has its own lock, which it may
acquire when being called.

Things get interesting when libxl calls back into ocaml code. There is a risk
of ending up in a deadlock when a thread holds both locks at the same time,
then temporarily drop the ocaml lock, while another thread calls another libxl
function.

To avoid deadlocks, we drop the ocaml heap lock before entering libxl, and
reacquire it in callbacks to ocaml. This way, the ocaml heap lock is never held
together with the libxl lock, except in osevent registration callbacks, and
xentoollog callbacks. If we guarantee to not call any libxl functions inside
those callbacks, we can avoid deadlocks.

This patch handle the dropping and reacquiring of the ocaml heap lock by the
caml_enter_blocking_section and caml_leave_blocking_section functions, and
related macros. We are also careful to not call any functions that access the
ocaml heap while the ocaml heap lock is dropped. This often involves copying
ocaml values to C before dropping the ocaml lock.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
libxl: ocaml: add some missing CAML macros

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
libxl: ocaml: fix memory corruption when converting string and key/values lists

Found by Coverty. CIDs: 1128562 1128563 1128564 1128565.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
libxl: ocaml: remove dead code in xentoollog bindings

Found by Coverty. CIDs: 1128567 1128568 1128576 1128577.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
libxl: ocaml: git/hgignore generated files

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>