Use irqfd when supported by kernel.
This uses msix mask notifiers: when vector is masked, we poll it from
userspace.  When it is unmasked, we poll it from kernel.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/virtio-pci.c |   31 +++++++++++++++++++++++++++++--
 1 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
index c454093..e8e0d82 100644
--- a/hw/virtio-pci.c
+++ b/hw/virtio-pci.c
@@ -404,6 +404,27 @@ static void virtio_pci_guest_notifier_read(void *opaque)
     }
 }
 
+static int virtio_pci_mask_notifier(PCIDevice *dev, unsigned vector,
+                                    void *opaque, int masked)
+{
+    VirtQueue *vq = opaque;
+    EventNotifier *notifier = virtio_queue_guest_notifier(vq);
+    int r = kvm_set_irqfd(dev->msix_irq_entries[vector].gsi,
+                          event_notifier_get_fd(notifier),
+                          !masked);
+    if (r < 0) {
+        return (r == -ENOSYS) ? 0 : r;
+    }
+    if (masked) {
+        qemu_set_fd_handler(event_notifier_get_fd(notifier),
+                            virtio_pci_guest_notifier_read, NULL, vq);
+    } else {
+        qemu_set_fd_handler(event_notifier_get_fd(notifier),
+                            NULL, NULL, vq);
+    }
+    return 0;
+}
+
 static int virtio_pci_guest_notifier(void *opaque, int n, bool assign)
 {
     VirtIOPCIProxy *proxy = opaque;
@@ -412,11 +433,15 @@ static int virtio_pci_guest_notifier(void *opaque, int n, bool assign)
 
     if (assign) {
         int r = event_notifier_init(notifier, 0);
-	if (r < 0)
-		return r;
+        if (r < 0)
+            return r;
         qemu_set_fd_handler(event_notifier_get_fd(notifier),
                             virtio_pci_guest_notifier_read, NULL, vq);
+        msix_set_mask_notifier(&proxy->pci_dev,
+			       virtio_queue_vector(proxy->vdev, n), vq);
     } else {
+        msix_set_mask_notifier(&proxy->pci_dev,
+			       virtio_queue_vector(proxy->vdev, n), NULL);
         qemu_set_fd_handler(event_notifier_get_fd(notifier),
                             NULL, NULL, vq);
         event_notifier_cleanup(notifier);
@@ -501,6 +526,8 @@ static void virtio_init_pci(VirtIOPCIProxy *proxy, VirtIODevice *vdev,
 
     proxy->pci_dev.config_write = virtio_write_config;
 
+    proxy->pci_dev.msix_mask_notifier = virtio_pci_mask_notifier;
+
     size = VIRTIO_PCI_REGION_SIZE(&proxy->pci_dev) + vdev->config_len;
     if (size & (size-1))
         size = 1 << qemu_fls(size);
-- 
1.6.6.144.g5c3af