x86/hvm: wait for at least one ioreq server to be enabled

In the case where a stub domain is providing emulation for an HVM
guest, there is no interlock in the toolstack to make sure that
the stub domain is up and running before the guest is unpaused.

Prior to the introduction of ioreq servers this was not a problem,
since there was only ever one emulator so ioreqs were simply
created anyway and the vcpu remained blocked until the stub domain
started and picked up the ioreq.

Since ioreq servers allow for multiple emulators for a single guest
it's not possible to know a priori which emulator will handle a
particular ioreq, so emulators must attach to a guest before the
guest runs.

This patch works around the lack of interlock in the toolstack for
stub domains by keeping the domain paused until at least one ioreq
server is created and enabled, which in practice means the stub
domain is indeed up and running.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
master commit: dd748d128d
master date: 2015-02-10 13:28:40 +0100

xen/arch/x86/hvm/hvm.c

@@ -874,6 +874,13 @@ static void hvm_ioreq_server_enable(struct hvm_ioreq_server *s,
 
   done:
     spin_unlock(&s->lock);
+
+    /* This check is protected by the domain ioreq server lock. */
+    if ( d->arch.hvm_domain.ioreq_server.waiting )
+    {
+        d->arch.hvm_domain.ioreq_server.waiting = 0;
+        domain_unpause(d);
+    }
 }
 
 static void hvm_ioreq_server_disable(struct hvm_ioreq_server *s,
@@ -1424,6 +1431,20 @@ int hvm_domain_initialise(struct domain *d)
 
     spin_lock_init(&d->arch.hvm_domain.ioreq_server.lock);
     INIT_LIST_HEAD(&d->arch.hvm_domain.ioreq_server.list);
+    
+    /*
+     * In the case where a stub domain is providing emulation for
+     * the guest, there is no interlock in the toolstack to prevent
+     * the guest from running before the stub domain is ready.
+     * Hence the domain must remain paused until at least one ioreq
+     * server is created and enabled.
+     */
+    if ( !is_pvh_domain(d) )
+    {
+        domain_pause(d);
+        d->arch.hvm_domain.ioreq_server.waiting = 1;
+    }
+
     spin_lock_init(&d->arch.hvm_domain.irq_lock);
     spin_lock_init(&d->arch.hvm_domain.uc_lock);
 

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

@@ -83,6 +83,7 @@ struct hvm_domain {
     struct {
         spinlock_t       lock;
         ioservid_t       id;
+        bool_t           waiting;
         struct list_head list;
     } ioreq_server;
     struct hvm_ioreq_server *default_ioreq_server;