[Xen-devel] [PATCHv1] evtchn: don't reuse ports that are still "busy"

[David Vrabel <david.vrabel@xxxxxxxxxx>]

When using the FIFO ABI a guest may close an event channel that is
still LINKED.  If this port is reused, subsequent events may be lost
because they may become pending on the wrong queue.

This could be fixed by requiring guests to only close event channels
that are not linked.  This is difficult since: a) irq cleanup in the
guest may be done in a context that cannot wait for the event to be
unlinked; b) the guest may attempt to rebind a PIRQ whose previous
close is still pending; and c) existing guests already have the
problematic behaviour.

Instead, simply check a port is not "busy" (i.e., it's not linked)
before reusing it.

Guests should still drain any queues for VCPUs that are being
offlined, or the port will become permanently unusable.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 xen/common/event_2l.c      |  7 +++++++
 xen/common/event_channel.c |  3 ++-
 xen/common/event_fifo.c    | 13 +++++++++++++
 xen/include/xen/event.h    | 11 +++++++++++
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/xen/common/event_2l.c b/xen/common/event_2l.c
index 5837ae8..0ccc59f 100644
--- a/xen/common/event_2l.c
+++ b/xen/common/event_2l.c
@@ -74,6 +74,12 @@ static bool_t evtchn_2l_is_masked(struct domain *d,
     return test_bit(evtchn->port, &shared_info(d, evtchn_mask));
 }
 
+static bool_t evtchn_2l_is_busy(struct domain *d,
+                                const struct evtchn *evtchn)
+{
+    return 0;
+}
+
 static void evtchn_2l_print_state(struct domain *d,
                                   const struct evtchn *evtchn)
 {
@@ -90,6 +96,7 @@ static const struct evtchn_port_ops evtchn_port_ops_2l =
     .unmask        = evtchn_2l_unmask,
     .is_pending    = evtchn_2l_is_pending,
     .is_masked     = evtchn_2l_is_masked,
+    .is_busy       = evtchn_2l_is_busy,
     .print_state   = evtchn_2l_print_state,
 };
 
diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index 5a529a6..b1d4e47 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -170,7 +170,8 @@ static int get_free_port(struct domain *d)
     {
         if ( port > d->max_evtchn_port )
             return -ENOSPC;
-        if ( evtchn_from_port(d, port)->state == ECS_FREE )
+        chn = evtchn_from_port(d, port);
+        if ( chn->state == ECS_FREE && !evtchn_port_is_busy(d, chn) )
             return port;
     }
 
diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index c9b7884..4157318 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -312,6 +312,18 @@ static bool_t evtchn_fifo_is_masked(struct domain *d,
     return test_bit(EVTCHN_FIFO_MASKED, word);
 }
 
+static bool_t evtchn_fifo_is_busy(struct domain *d,
+                                  const struct evtchn *evtchn)
+{
+    event_word_t *word;
+
+    word = evtchn_fifo_word_from_port(d, evtchn->port);
+    if ( unlikely(!word) )
+        return 0;
+
+    return test_bit(EVTCHN_FIFO_LINKED, word);
+}
+
 static int evtchn_fifo_set_priority(struct domain *d, struct evtchn *evtchn,
                                     unsigned int priority)
 {
@@ -351,6 +363,7 @@ static const struct evtchn_port_ops evtchn_port_ops_fifo =
     .unmask        = evtchn_fifo_unmask,
     .is_pending    = evtchn_fifo_is_pending,
     .is_masked     = evtchn_fifo_is_masked,
+    .is_busy       = evtchn_fifo_is_busy,
     .set_priority  = evtchn_fifo_set_priority,
     .print_state   = evtchn_fifo_print_state,
 };
diff --git a/xen/include/xen/event.h b/xen/include/xen/event.h
index b87924a..b754f08 100644
--- a/xen/include/xen/event.h
+++ b/xen/include/xen/event.h
@@ -139,6 +139,11 @@ struct evtchn_port_ops {
     void (*unmask)(struct domain *d, struct evtchn *evtchn);
     bool_t (*is_pending)(struct domain *d, const struct evtchn *evtchn);
     bool_t (*is_masked)(struct domain *d, const struct evtchn *evtchn);
+    /*
+     * Is the port unavailable because its still being cleaned up
+     * after being closed?
+     */
+    bool_t (*is_busy)(struct domain *d, const struct evtchn *evtchn);
     int (*set_priority)(struct domain *d, struct evtchn *evtchn,
                         unsigned int priority);
     void (*print_state)(struct domain *d, const struct evtchn *evtchn);
@@ -181,6 +186,12 @@ static inline bool_t evtchn_port_is_masked(struct domain 
*d,
     return d->evtchn_port_ops->is_masked(d, evtchn);
 }
 
+static inline bool_t evtchn_port_is_busy(struct domain *d,
+                                         const struct evtchn *evtchn)
+{
+    return d->evtchn_port_ops->is_busy(d, evtchn);
+}
+
 static inline int evtchn_port_set_priority(struct domain *d,
                                            struct evtchn *evtchn,
                                            unsigned int priority)
-- 
2.1.4


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