x86/hvm: take a reference on ioreq server emulating domain

When an ioreq server is created the code currently stores the id
of the emulating domain, but does not take a reference on that domain.

This patch modifies the code to hold a reference for the lifetime of the
ioreq server.

NOTE: ioreq servers are either destroyed explicitly or destroyed implicitly
      in context of XEN_DOMCTL_destroydomain.
      If the emulating domain is shut down prior to the target then the
      any domain reference held by an ioreq server will prevent it from
      being destroyed. However, if an emulating domain is shut down prior
      to its target then it is likely that the target's vcpus will block
      fairly quickly waiting for emulation that will never occur, and when
      the target domain is destroyed the reference on the zombie emulating
      domain will be dropped allowing both to be cleaned up.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/hvm/ioreq.c

@@ -218,7 +218,7 @@ static void hvm_unmap_ioreq_page(struct hvm_ioreq_server *s, bool buf)
 static int hvm_map_ioreq_page(
     struct hvm_ioreq_server *s, bool buf, unsigned long gfn)
 {
-    struct domain *d = s->domain;
+    struct domain *d = s->target;
     struct hvm_ioreq_page *iorp = buf ? &s->bufioreq : &s->ioreq;
     struct page_info *page;
     void *va;
@@ -315,8 +315,8 @@ static int hvm_ioreq_server_add_vcpu(struct hvm_ioreq_server *s,
 
     spin_lock(&s->lock);
 
-    rc = alloc_unbound_xen_event_channel(v->domain, v->vcpu_id, s->domid,
-                                         NULL);
+    rc = alloc_unbound_xen_event_channel(v->domain, v->vcpu_id,
+                                         s->emulator->domain_id, NULL);
     if ( rc < 0 )
         goto fail2;
 
@@ -324,9 +324,10 @@ static int hvm_ioreq_server_add_vcpu(struct hvm_ioreq_server *s,
 
     if ( v->vcpu_id == 0 && s->bufioreq.va != NULL )
     {
-        struct domain *d = s->domain;
+        struct domain *d = s->target;
 
-        rc = alloc_unbound_xen_event_channel(v->domain, 0, s->domid, NULL);
+        rc = alloc_unbound_xen_event_channel(v->domain, 0,
+                                             s->emulator->domain_id, NULL);
         if ( rc < 0 )
             goto fail3;
 
@@ -434,7 +435,7 @@ static int hvm_ioreq_server_setup_pages(struct hvm_ioreq_server *s,
                                         bool is_default,
                                         bool handle_bufioreq)
 {
-    struct domain *d = s->domain;
+    struct domain *d = s->target;
     unsigned long ioreq_gfn = gfn_x(INVALID_GFN);
     unsigned long bufioreq_gfn = gfn_x(INVALID_GFN);
     int rc;
@@ -471,7 +472,7 @@ static int hvm_ioreq_server_setup_pages(struct hvm_ioreq_server *s,
 static void hvm_ioreq_server_unmap_pages(struct hvm_ioreq_server *s,
                                          bool is_default)
 {
-    struct domain *d = s->domain;
+    struct domain *d = s->target;
     bool handle_bufioreq = !!s->bufioreq.va;
 
     if ( handle_bufioreq )
@@ -521,7 +522,7 @@ static int hvm_ioreq_server_alloc_rangesets(struct hvm_ioreq_server *s,
         if ( rc )
             goto fail;
 
-        s->range[i] = rangeset_new(s->domain, name,
+        s->range[i] = rangeset_new(s->target, name,
                                    RANGESETF_prettyprint_hex);
 
         xfree(name);
@@ -545,7 +546,7 @@ static int hvm_ioreq_server_alloc_rangesets(struct hvm_ioreq_server *s,
 static void hvm_ioreq_server_enable(struct hvm_ioreq_server *s,
                                     bool is_default)
 {
-    struct domain *d = s->domain;
+    struct domain *d = s->target;
     struct hvm_ioreq_vcpu *sv;
     bool handle_bufioreq = !!s->bufioreq.va;
 
@@ -576,7 +577,7 @@ static void hvm_ioreq_server_enable(struct hvm_ioreq_server *s,
 static void hvm_ioreq_server_disable(struct hvm_ioreq_server *s,
                                      bool is_default)
 {
-    struct domain *d = s->domain;
+    struct domain *d = s->target;
     bool handle_bufioreq = !!s->bufioreq.va;
 
     spin_lock(&s->lock);
@@ -602,12 +603,15 @@ static int hvm_ioreq_server_init(struct hvm_ioreq_server *s,
                                  struct domain *d, bool is_default,
                                  int bufioreq_handling, ioservid_t id)
 {
+    struct domain *currd = current->domain;
     struct vcpu *v;
     int rc;
 
     s->id = id;
-    s->domain = d;
-    s->domid = current->domain->domain_id;
+    s->target = d;
+
+    get_knownalive_domain(currd);
+    s->emulator = currd;
 
     spin_lock_init(&s->lock);
     INIT_LIST_HEAD(&s->ioreq_vcpu_list);
@@ -641,6 +645,7 @@ static int hvm_ioreq_server_init(struct hvm_ioreq_server *s,
  fail_map:
     hvm_ioreq_server_free_rangesets(s, is_default);
 
+    put_domain(s->emulator);
     return rc;
 }
 
@@ -651,6 +656,8 @@ static void hvm_ioreq_server_deinit(struct hvm_ioreq_server *s,
     hvm_ioreq_server_remove_all_vcpus(s);
     hvm_ioreq_server_unmap_pages(s, is_default);
     hvm_ioreq_server_free_rangesets(s, is_default);
+
+    put_domain(s->emulator);
 }
 
 static ioservid_t next_ioservid(struct domain *d)

xen/include/asm-x86/hvm/domain.h

@@ -53,13 +53,11 @@ struct hvm_ioreq_vcpu {
 
 struct hvm_ioreq_server {
     struct list_head       list_entry;
-    struct domain          *domain;
+    struct domain          *target, *emulator;
 
     /* Lock to serialize toolstack modifications */
     spinlock_t             lock;
 
-    /* Domain id of emulating domain */
-    domid_t                domid;
     ioservid_t             id;
     struct hvm_ioreq_page  ioreq;
     struct list_head       ioreq_vcpu_list;