xen/events: access last_priority and last_vcpu_id together

The queue for a fifo event is depending on the vcpu_id and the
priority of the event. When sending an event it might happen the
event needs to change queues and the old queue needs to be kept for
keeping the links between queue elements intact. For this purpose
the event channel contains last_priority and last_vcpu_id values
elements for being able to identify the old queue.

In order to avoid races always access last_priority and last_vcpu_id
with a single atomic operation avoiding any inconsistencies.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
master commit: 1277cb9dc5e966f1faf665bcded02b7533e38078
master date: 2020-11-24 11:23:42 +0100

diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 27ab3a1c3f161436039aee2ab2753988f227d08f..2037b2419674a93e28ae1c56a2ae745a96b714d7 100644 (file)
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -21,6 +21,14 @@
 
 #include <public/event_channel.h>
 
+union evtchn_fifo_lastq {
+    uint32_t raw;
+    struct {
+        uint8_t last_priority;
+        uint16_t last_vcpu_id;
+    };
+};
+
 static inline event_word_t *evtchn_fifo_word_from_port(const struct domain *d,
                                                        unsigned int port)
 {
@@ -65,16 +73,18 @@ static struct evtchn_fifo_queue *lock_old_queue(const struct domain *d,
     struct vcpu *v;
     struct evtchn_fifo_queue *q, *old_q;
     unsigned int try;
+    union evtchn_fifo_lastq lastq;
 
     for ( try = 0; try < 3; try++ )
     {
-        v = d->vcpu[evtchn->last_vcpu_id];
-        old_q = &v->evtchn_fifo->queue[evtchn->last_priority];
+        lastq.raw = read_atomic(&evtchn->fifo_lastq);
+        v = d->vcpu[lastq.last_vcpu_id];
+        old_q = &v->evtchn_fifo->queue[lastq.last_priority];
 
         spin_lock_irqsave(&old_q->lock, *flags);
 
-        v = d->vcpu[evtchn->last_vcpu_id];
-        q = &v->evtchn_fifo->queue[evtchn->last_priority];
+        v = d->vcpu[lastq.last_vcpu_id];
+        q = &v->evtchn_fifo->queue[lastq.last_priority];
 
         if ( old_q == q )
             return old_q;
@@ -225,8 +235,11 @@ static void evtchn_fifo_set_pending(struct vcpu *v, struct evtchn *evtchn)
         /* Moved to a different queue? */
         if ( old_q != q )
         {
-            evtchn->last_vcpu_id = v->vcpu_id;
-            evtchn->last_priority = q->priority;
+            union evtchn_fifo_lastq lastq = { };
+
+            lastq.last_vcpu_id = v->vcpu_id;
+            lastq.last_priority = q->priority;
+            write_atomic(&evtchn->fifo_lastq, lastq.raw);
 
             spin_unlock_irqrestore(&old_q->lock, flags);
             spin_lock_irqsave(&q->lock, flags);

diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index f782ffeb828a12bbf050f0b2a5f9c3fb675155a9..99e2f1aac582d7bfdd661dedc9b789856d410718 100644 (file)
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -117,8 +117,7 @@ struct evtchn
 #ifndef NDEBUG
     u8 old_state;      /* State when taking lock in write mode. */
 #endif
-    u8 last_priority;
-    u16 last_vcpu_id;
+    u32 fifo_lastq;    /* Data for fifo events identifying last queue. */
 #ifdef CONFIG_XSM
     union {
 #ifdef XSM_NEED_GENERIC_EVTCHN_SSID