[Qemu-devel] [RFC PATCH] virtio: set ISR status when delivering queue MSI

[Ladi Prosek <lprosek@redhat.com>]

There is a discrepancy between dataplane and no-dataplane virtio
behavior with respect to the ISR status register and MSI-X
capability.

Without dataplane the Queue interrupt ISR status bit is set
regardless of how the notification is delivered to the guest.

With dataplane the Queue interrupt ISR status bit is set only
if the notification is delivered as an IRQ.

While both conform to the spec, QEMU should be consistent to
minimize surprises with broken drivers.

This RFC patch shows the relevant code location and contains a
possible fix which makes QEMU set the bit on the MSI dataplane
code path.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
---
 hw/virtio/virtio.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 08275a9..4053313 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1192,7 +1192,7 @@ void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
     }
 
     trace_virtio_notify(vdev, vq);
-    vdev->isr |= 0x01;
+    vdev->isr |= 0x01; // <= here we set ISR status even if we don't raise IRQ
     virtio_notify_vector(vdev, vq->vector);
 }
 
@@ -1757,16 +1757,25 @@ static void virtio_queue_guest_notifier_read(EventNotifier *n)
     }
 }
 
+static void virtio_queue_guest_notifier_read_irqfd(EventNotifier *n)
+{
+    VirtQueue *vq = container_of(n, VirtQueue, guest_notifier);
+    if (event_notifier_test_and_clear(n)) {
+        vq->vdev->isr |= 0x01;
+    }
+}
+
 void virtio_queue_set_guest_notifier_fd_handler(VirtQueue *vq, bool assign,
                                                 bool with_irqfd)
 {
-    if (assign && !with_irqfd) {
+    if (assign) {
         event_notifier_set_handler(&vq->guest_notifier,
-                                   virtio_queue_guest_notifier_read);
+             with_irqfd ?
+                virtio_queue_guest_notifier_read_irqfd :
+                virtio_queue_guest_notifier_read);
     } else {
         event_notifier_set_handler(&vq->guest_notifier, NULL);
-    }
-    if (!assign) {
+
         /* Test and clear notifier before closing it,
          * in case poll callback didn't have time to run. */
         virtio_queue_guest_notifier_read(&vq->guest_notifier);
-- 
2.5.5