x86/hvm: don't unconditionally create a default ioreq server

Avoid doing so if the domain is not under construction.

If upstream QEMU is in use then it will explicitly create an ioreq server
rather than implicitly creating the default ioreq server, which is a
side-effect of reading HVM_PARAM_IOREQ_PFN, HVM_PARAM_BUFIOREQ_PFN,
or HVM_PARAM_BUFIOREQ_EVTCHN (as is done by legacy QEMUs).

However, if the domain is subsequently saved/migrated then those parameters
are read and hence the default server will be unnecessarily instantiated.

This patch adds an extra check of the 'creation_finished' flag when those
HVM params are read and will only instantiate the server if the domain is
under construction, which will always be the case when QEMU is invoked.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/hvm.c

@@ -5278,7 +5278,16 @@ static int hvmop_get_param(
     {
         domid_t domid;
 
-        /* May need to create server. */
+        /*
+         * It may be necessary to create a default ioreq server here,
+         * because legacy versions of QEMU are not aware of the new API for
+         * explicit ioreq server creation. However, if the domain is not
+         * under construction then it will not be QEMU querying the
+         * parameters and thus the query should not have that side-effect.
+         */
+        if ( d->creation_finished )
+            break;
+
         domid = d->arch.hvm_domain.params[HVM_PARAM_DM_DOMAIN];
         rc = hvm_create_ioreq_server(d, domid, 1,
                                      HVM_IOREQSRV_BUFIOREQ_LEGACY, NULL);