xen/ioreq: Introduce domain_has_ioreq_server()

This patch introduces a helper the main purpose of which is to check
if a domain is using IOREQ server(s).

On Arm the current benefit is to avoid calling vcpu_ioreq_handle_completion()
(which implies iterating over all possible IOREQ servers anyway)
on every return in leave_hypervisor_to_guest() if there is no active
servers for the particular domain.
Also this helper will be used by one of the subsequent patches on Arm.

This involves adding an extra per-domain variable to store the count
of servers in use.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
CC: Julien Grall <julien.grall@arm.com>

Message-Id: <1606732298-22107-18-git-send-email-olekstysh@gmail.com>

4 files changed, 30 insertions, 7 deletions

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 4cef43e897..b6077d2b0c 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -2262,14 +2262,17 @@ static bool check_for_vcpu_work(void)
     struct vcpu *v = current;
 
 #ifdef CONFIG_IOREQ_SERVER
-    bool handled;
+    if ( domain_has_ioreq_server(v->domain) )
+    {
+        bool handled;
 
-    local_irq_enable();
-    handled = vcpu_ioreq_handle_completion(v);
-    local_irq_disable();
+        local_irq_enable();
+        handled = vcpu_ioreq_handle_completion(v);
+        local_irq_disable();
 
-    if ( !handled )
-        return true;
+        if ( !handled )
+            return true;
+    }
 #endif
 
     if ( likely(!v->arch.need_flush_to_ram) )
diff --git a/xen/common/ioreq.c b/xen/common/ioreq.c
index 4855dd8362..f35dcf9810 100644
--- a/xen/common/ioreq.c
+++ b/xen/common/ioreq.c
@@ -39,9 +39,14 @@ static void set_ioreq_server(struct domain *d, unsigned int id,
                              struct ioreq_server *s)
 {
     ASSERT(id < MAX_NR_IOREQ_SERVERS);
-    ASSERT(!s || !d->ioreq_server.server[id]);
+    ASSERT(!s ^ !d->ioreq_server.server[id]);
 
     d->ioreq_server.server[id] = s;
+
+    if ( s )
+        d->ioreq_server.nr_servers++;
+    else
+        d->ioreq_server.nr_servers--;
 }
 
 #define GET_IOREQ_SERVER(d, id) \
diff --git a/xen/include/xen/ioreq.h b/xen/include/xen/ioreq.h
index 02ff998c57..2289e79aac 100644
--- a/xen/include/xen/ioreq.h
+++ b/xen/include/xen/ioreq.h
@@ -55,6 +55,20 @@ struct ioreq_server {
     uint8_t                bufioreq_handling;
 };
 
+/*
+ * This should only be used when d == current->domain and it's not paused,
+ * or when they're distinct and d is paused. Otherwise the result is
+ * stale before the caller can inspect it.
+ */
+static inline bool domain_has_ioreq_server(const struct domain *d)
+{
+#ifdef CONFIG_IOREQ_SERVER
+    return d->ioreq_server.nr_servers;
+#else
+    return false;
+#endif
+}
+
 static inline paddr_t ioreq_mmio_first_byte(const ioreq_t *p)
 {
     return unlikely(p->df) ?
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 8269f84a37..2277995045 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -550,6 +550,7 @@ struct domain
     struct {
         spinlock_t              lock;
         struct ioreq_server     *server[MAX_NR_IOREQ_SERVERS];
+        unsigned int            nr_servers;
     } ioreq_server;
 #endif
 };