xen/arm: optee: add support for arbitrary shared memory

xen/arm: optee: add support for arbitrary shared memory

Shared memory is widely used by NW (Normal World) to communicate with
TAs (Trusted Applications) in OP-TEE. NW can share part of own memory
with TA or with OP-TEE core, by registering it in OP-TEE, or by
providing a temporal reference. Anyways, information about such memory
buffers are sent to OP-TEE as a list of pages. This mechanism is
described in optee_msg.h.

Mediator should step in when NW tries to share memory with
OP-TEE for two reasons:

1. Do address translation from IPA to PA.
2. Pin domain pages while they are mapped into OP-TEE or TA
   address space, so domain can't transfer this pages to
   other domain or balloon out them.

Address translation is done by translate_noncontig(...) function.
It allocates new buffer from domheap and then walks on guest
provided list of pages, translates addresses and stores PAs into
newly allocated buffer. This buffer will be provided to OP-TEE
instead of original buffer from the guest. This buffer will
be freed at the end of standard call.

In the same time this function pins pages and stores them in
struct optee_shm_buf object. This object will live all the time,
when given SHM buffer is known to OP-TEE. It will be freed
after guest unregisters shared buffer. At this time pages
will be unpinned.

Guest can share buffer with OP-TEE for duration for one call,
or permanently, using OPTEE_MSG_CMD_REGISTER_SHM call. We need
to handle both options.

Also we want to limit total size of shared buffers. As it is not
possible to get limit from OP-TEE, we need to choose some arbitrary
value. Currently limit is 16384 of 4K pages.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Julien Grall <julien.grall@arm.com>