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.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
CC: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Paul Durrant <paul@xen.org>
[On Arm only]
Tested-by: Wei Chen <Wei.Chen@arm.com>

xen/arch/arm/traps.c

@@ -2275,14 +2275,17 @@ 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) )

xen/common/ioreq.c

@@ -80,6 +80,22 @@ static ioreq_t *get_ioreq(struct ioreq_server *s, struct vcpu *v)
     return &p->vcpu_ioreq[v->vcpu_id];
 }
 
+/*
+ * This should only be used when d == current->domain or when they're
+ * distinct and d is paused. Otherwise the result is stale before
+ * the caller can inspect it.
+ */
+bool domain_has_ioreq_server(const struct domain *d)
+{
+    const struct ioreq_server *s;
+    unsigned int id;
+
+    FOR_EACH_IOREQ_SERVER(d, id, s)
+        return true;
+
+    return false;
+}
+
 static struct ioreq_vcpu *get_pending_vcpu(const struct vcpu *v,
                                            struct ioreq_server **srvp)
 {

xen/include/xen/ioreq.h

@@ -83,6 +83,8 @@ static inline bool ioreq_needs_completion(const ioreq_t *ioreq)
 #define HANDLE_BUFIOREQ(s) \
     ((s)->bufioreq_handling != HVM_IOREQSRV_BUFIOREQ_OFF)
 
+bool domain_has_ioreq_server(const struct domain *d);
+
 bool vcpu_ioreq_pending(struct vcpu *v);
 bool vcpu_ioreq_handle_completion(struct vcpu *v);
 bool is_ioreq_server_page(struct domain *d, const struct page_info *page);