virtio: set ISR on dataplane notifications

Dataplane has been omitting forever the step of setting ISR when
an interrupt is raised.  This caused little breakage, because the
specification actually says that ISR may not be updated in MSI mode.

Some versions of the Windows drivers however didn't clear MSI mode
correctly, and proceeded using polling mode (using ISR, not the used
ring index!) for crashdump and hibernation.  If it were just crashdump
and hibernation it would not be a big deal, but recent releases of
Windows do not really shut down, but rather log out and hibernate to
make the next startup faster.  Hence, this manifested as a more serious
hang during shutdown with e.g. Windows 8.1 and virtio-win 1.8.0 RPMs.
Newer versions fixed this, while older versions do not use MSI at all.

The failure has always been there for virtio dataplane, but it became
visible after commits 9ffe337 ("virtio-blk: always use dataplane path
if ioeventfd is active", 2016-10-30) and ad07cd6 ("virtio-scsi: always
use dataplane path if ioeventfd is active", 2016-10-30) made virtio-blk
and virtio-scsi always use the dataplane code under KVM.  The good news
therefore is that it was not a bug in the patches---they were doing
exactly what they were meant for, i.e. shake out remaining dataplane bugs.

The fix is not hard, so it's worth arranging for the broken drivers.
The virtio_should_notify+event_notifier_set pair that is common to
virtio-blk and virtio-scsi dataplane is replaced with a new public
function virtio_notify_irqfd that also sets ISR.  The irqfd emulation
code now need not set ISR anymore, so virtio_irq is removed.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -68,9 +68,7 @@ static void notify_guest_bh(void *opaque)
             unsigned i = j + ctzl(bits);
             VirtQueue *vq = virtio_get_queue(s->vdev, i);
 
-            if (virtio_should_notify(s->vdev, vq)) {
-                event_notifier_set(virtio_queue_get_guest_notifier(vq));
-            }
+            virtio_notify_irqfd(s->vdev, vq);
 
             bits &= bits - 1; /* clear right-most bit */
         }

hw/scsi/virtio-scsi-dataplane.c

@@ -95,13 +95,6 @@ static int virtio_scsi_vring_init(VirtIOSCSI *s, VirtQueue *vq, int n,
     return 0;
 }
 
-void virtio_scsi_dataplane_notify(VirtIODevice *vdev, VirtIOSCSIReq *req)
-{
-    if (virtio_should_notify(vdev, req->vq)) {
-        event_notifier_set(virtio_queue_get_guest_notifier(req->vq));
-    }
-}
-
 /* assumes s->ctx held */
 static void virtio_scsi_clear_aio(VirtIOSCSI *s)
 {

hw/scsi/virtio-scsi.c

@@ -69,7 +69,7 @@ static void virtio_scsi_complete_req(VirtIOSCSIReq *req)
     qemu_iovec_from_buf(&req->resp_iov, 0, &req->resp, req->resp_size);
     virtqueue_push(vq, &req->elem, req->qsgl.size + req->resp_iov.size);
     if (s->dataplane_started && !s->dataplane_fenced) {
-        virtio_scsi_dataplane_notify(vdev, req);
+        virtio_notify_irqfd(vdev, vq);
     } else {
         virtio_notify(vdev, vq);
     }

hw/virtio/trace-events

@@ -5,7 +5,7 @@ virtqueue_fill(void *vq, const void *elem, unsigned int len, unsigned int idx) "
 virtqueue_flush(void *vq, unsigned int count) "vq %p count %u"
 virtqueue_pop(void *vq, void *elem, unsigned int in_num, unsigned int out_num) "vq %p elem %p in_num %u out_num %u"
 virtio_queue_notify(void *vdev, int n, void *vq) "vdev %p n %d vq %p"
-virtio_irq(void *vq) "vq %p"
+virtio_notify_irqfd(void *vdev, void *vq) "vdev %p vq %p"
 virtio_notify(void *vdev, void *vq) "vdev %p vq %p"
 virtio_set_status(void *vdev, uint8_t val) "vdev %p val %u"
 

hw/virtio/virtio.c

@@ -1330,13 +1330,6 @@ static void virtio_set_isr(VirtIODevice *vdev, int value)
     }
 }
 
-void virtio_irq(VirtQueue *vq)
-{
-    trace_virtio_irq(vq);
-    virtio_set_isr(vq->vdev, 0x1);
-    virtio_notify_vector(vq->vdev, vq->vector);
-}
-
 bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
 {
     uint16_t old, new;
@@ -1360,6 +1353,33 @@ bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
     return !v || vring_need_event(vring_get_used_event(vq), new, old);
 }
 
+void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
+{
+    if (!virtio_should_notify(vdev, vq)) {
+        return;
+    }
+
+    trace_virtio_notify_irqfd(vdev, vq);
+
+    /*
+     * virtio spec 1.0 says ISR bit 0 should be ignored with MSI, but
+     * windows drivers included in virtio-win 1.8.0 (circa 2015) are
+     * incorrectly polling this bit during crashdump and hibernation
+     * in MSI mode, causing a hang if this bit is never updated.
+     * Recent releases of Windows do not really shut down, but rather
+     * log out and hibernate to make the next startup faster.  Hence,
+     * this manifested as a more serious hang during shutdown with
+     *
+     * Next driver release from 2016 fixed this problem, so working around it
+     * is not a must, but it's easy to do so let's do it here.
+     *
+     * Note: it's safe to update ISR from any thread as it was switched
+     * to an atomic operation.
+     */
+    virtio_set_isr(vq->vdev, 0x1);
+    event_notifier_set(&vq->guest_notifier);
+}
+
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
 {
     if (!virtio_should_notify(vdev, vq)) {
@@ -1994,7 +2014,7 @@ static void virtio_queue_guest_notifier_read(EventNotifier *n)
 {
     VirtQueue *vq = container_of(n, VirtQueue, guest_notifier);
     if (event_notifier_test_and_clear(n)) {
-        virtio_irq(vq);
+        virtio_notify_vector(vq->vdev, vq->vector);
     }
 }
 

include/hw/virtio/virtio-scsi.h

@@ -137,6 +137,5 @@ void virtio_scsi_push_event(VirtIOSCSI *s, SCSIDevice *dev,
 void virtio_scsi_dataplane_setup(VirtIOSCSI *s, Error **errp);
 int virtio_scsi_dataplane_start(VirtIODevice *s);
 void virtio_scsi_dataplane_stop(VirtIODevice *s);
-void virtio_scsi_dataplane_notify(VirtIODevice *vdev, VirtIOSCSIReq *req);
 
 #endif /* QEMU_VIRTIO_SCSI_H */

include/hw/virtio/virtio.h

@@ -181,6 +181,7 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
                                unsigned max_in_bytes, unsigned max_out_bytes);
 
 bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq);
+void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq);
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);
 
 void virtio_save(VirtIODevice *vdev, QEMUFile *f);
@@ -280,7 +281,6 @@ void virtio_queue_host_notifier_read(EventNotifier *n);
 void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
                                                 void (*fn)(VirtIODevice *,
                                                            VirtQueue *));
-void virtio_irq(VirtQueue *vq);
 VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);
 VirtQueue *virtio_vector_next_queue(VirtQueue *vq);