x86/HVM: do not retry in hvmemul_do_io() if no ioreq server exists for this I/O

x86/HVM: do not retry in hvmemul_do_io() if no ioreq server exists for this I/O

This saves a VMENTRY and a VMEXIT since we no longer retry the
ioport read on backing DM not handling a given ioreq.

There are 2 case about "no ioreq server exists for this I/O":

1) No ioreq servers (PVH case)
2) No ioreq servers for this I/O (non PVH case)

The routine hvm_has_dm() used to check for the empty list, the PVH
case (#1).

By changing from hvm_has_dm() to hvm_select_ioreq_server() both
cases are considered.  Doing it this way allows
hvm_send_assist_req() to only have 2 possible return values.

The key part of skipping the retry is to do "rc = X86EMUL_OKAY"
which is what the error path on the call to hvm_has_dm() does in
hvmemul_do_io() (the only call on hvm_has_dm()).

Since this case is no longer handled in hvm_send_assist_req(), move
the call to hvm_complete_assist_req() into hvmemul_do_io().

As part of this change, do the work of hvm_complete_assist_req() in
the PVH case.  Acting more like real hardware looks to be better.

Adding "rc = X86EMUL_OKAY" in the failing case of
hvm_send_assist_req() would break what was done in commit
bac0999325056a3b3a92f7622df7ffbc5388b1c3 and commit
f20f3c8ece5c10fa7626f253d28f570a43b23208.  We are currently doing
the succeeding case of hvm_send_assist_req() and retying the I/O.

Since hvm_select_ioreq_server() has already been called, switch to
using hvm_send_assist_req_to_ioreq_server().

Since there is no longer any calls to hvm_send_assist_req(), drop
that routine and rename hvm_send_assist_req_to_ioreq_server() to
hvm_send_assist_req.

Since hvm_send_assist_req() is an extern, add an ASSERT() on s.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>