[PATCH V3 17/23] xen/ioreq: Introduce domain_has_ioreq_server()

[Oleksandr Tyshchenko <olekstysh@gmail.com>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

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>

---
Please note, this is a split/cleanup/hardening of Julien's PoC:
"Add support for Guest IO forwarding to a device emulator"

Changes RFC -> V1:
   - new patch

Changes V1 -> V2:
   - update patch description
   - guard helper with CONFIG_IOREQ_SERVER
   - remove "hvm" prefix
   - modify helper to just return d->arch.hvm.ioreq_server.nr_servers
   - put suitable ASSERT()s
   - use ASSERT(d->ioreq_server.server[id] ? !s : !!s) in set_ioreq_server()
   - remove d->ioreq_server.nr_servers = 0 from hvm_ioreq_init()

Changes V2 -> V3:
   - update patch description
   - remove ASSERT()s from the helper, add a comment
   - use #ifdef CONFIG_IOREQ_SERVER inside function body
   - use new ASSERT() construction in set_ioreq_server()
---
---
 xen/arch/arm/traps.c    | 15 +++++++++------
 xen/common/ioreq.c      |  7 ++++++-
 xen/include/xen/ioreq.h | 14 ++++++++++++++
 xen/include/xen/sched.h |  1 +
 4 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 4cef43e..b6077d2 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 4855dd8..f35dcf9 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 02ff998..2289e79 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 8269f84..2277995 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
 };
-- 
2.7.4