argo: init, destroy and soft-reset, with enable command line opt

argo: init, destroy and soft-reset, with enable command line opt

Initialises basic data structures and performs teardown of argo state
for domain shutdown.

Inclusion of the Argo implementation is dependent on CONFIG_ARGO.

Introduces a new Xen command line parameter 'argo': bool to enable/disable
the argo hypercall. Defaults to disabled.

New headers:
  public/argo.h: with definions of addresses and ring structure, including
  indexes for atomic update for communication between domain and hypervisor.

  xen/argo.h: to expose the hooks for integration into domain lifecycle:
    argo_init: per-domain init of argo data structures for domain_create.
    argo_destroy: teardown for domain_destroy and the error exit
                  path of domain_create.
    argo_soft_reset: reset of domain state for domain_soft_reset.

Adds a new field to struct domain: struct argo_domain *argo;

In accordance with recent work on _domain_destroy, argo_destroy is
idempotent. It will tear down: all rings registered by this domain, all
rings where this domain is the single sender (ie. specified partner,
non-wildcard rings), and all pending notifications where this domain is
awaiting signal about available space in the rings of other domains.

A count will be maintained of the number of rings that a domain has
registered in order to limit it below the fixed maximum limit defined here.

Macros are defined to verify the internal locking state within the argo
implementation. The macros are ASSERTed on entry to functions to validate
and document the required lock state prior to calling.

The hash function for the hashtables that hold ring state is derived from
the string hashing function djb2 (http://www.cse.yorku.ca/~oz/hash.html)
by Daniel J. Bernstein. Basic testing with a limited number of domains and
ports has shown reasonable distribution for the table size.

The software license on the public header is the BSD license, standard
procedure for the public Xen headers. The public header was originally
posted under a GPL license at: [1]:
https://lists.xenproject.org/archives/html/xen-devel/2013-05/msg02710.html

The following ACK by Lars Kurth is to confirm that only people being
employees of Citrix contributed to the header files in the series posted at
[1] and that thus the copyright of the files in question is fully owned by
Citrix. The ACK also confirms that Citrix is happy for the header files to
be published under a BSD license in this series (which is based on [1]).

Signed-off-by: Christopher Clark <christopher.clark6@baesystems.com>
Acked-by: Lars Kurth <lars.kurth@citrix.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>
Tested-by: Chris Patterson <pattersonc@ainfosec.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>