[xen stable-4.10] xen/events: access last_priority and last_vcpu_id together

[patchbot@xxxxxxx]

commit 8eb5328e2ec6aa15e371ab64348265b26f00cdf5
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Tue Dec 1 17:36:41 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Dec 1 17:36:41 2020 +0100

    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@xxxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
    master commit: 1277cb9dc5e966f1faf665bcded02b7533e38078
    master date: 2020-11-24 11:23:42 +0100
---
 xen/common/event_fifo.c | 25 +++++++++++++++++++------
 xen/include/xen/sched.h |  3 +--
 2 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 98742ba9cb..b1951a29ad 100644
--- 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)
 {
@@ -64,16 +72,18 @@ 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;
@@ -224,8 +234,11 @@ 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 800b50289d..88740cd2ca 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -113,8 +113,7 @@ 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
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.10