[Qemu-devel] [PATCH 3/3] virtio-pci: don't poll masked vectors

["Michael S. Tsirkin" <mst@redhat.com>]

At the moment, when irqfd is in use but a vector is masked,
qemu will poll it and handle vector masks in userspace.
Since almost no one ever looks at the pending bits,
it is better to defer this until pending bits
are actually read.
Implement this optimization using the new poll notifier.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/virtio-pci.c | 52 ++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 40 insertions(+), 12 deletions(-)

diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
index 1c03bb5..bc6b4e0 100644
--- a/hw/virtio-pci.c
+++ b/hw/virtio-pci.c
@@ -509,8 +509,6 @@ static int kvm_virtio_pci_vq_vector_use(VirtIOPCIProxy *proxy,
         }
         return ret;
     }
-
-    virtio_queue_set_guest_notifier_fd_handler(vq, true, true);
     return 0;
 }
 
@@ -529,8 +527,6 @@ static void kvm_virtio_pci_vq_vector_release(VirtIOPCIProxy *proxy,
     if (--irqfd->users == 0) {
         kvm_irqchip_release_virq(kvm_state, irqfd->virq);
     }
-
-    virtio_queue_set_guest_notifier_fd_handler(vq, true, false);
 }
 
 static int kvm_virtio_pci_vector_use(PCIDevice *dev, unsigned vector,
@@ -581,7 +577,36 @@ static void kvm_virtio_pci_vector_release(PCIDevice *dev, unsigned vector)
     }
 }
 
-static int virtio_pci_set_guest_notifier(DeviceState *d, int n, bool assign)
+static void kvm_virtio_pci_vector_poll(PCIDevice *dev,
+                                       unsigned int vector_start,
+                                       unsigned int vector_end)
+{
+    VirtIOPCIProxy *proxy = container_of(dev, VirtIOPCIProxy, pci_dev);
+    VirtIODevice *vdev = proxy->vdev;
+    int queue_no;
+    unsigned int vector;
+    EventNotifier *notifier;
+    VirtQueue *vq;
+
+    for (queue_no = 0; queue_no < VIRTIO_PCI_QUEUE_MAX; queue_no++) {
+        if (!virtio_queue_get_num(vdev, queue_no)) {
+            break;
+        }
+        vector = virtio_queue_vector(vdev, queue_no);
+        if (vector < vector_start || vector >= vector_end ||
+            !msix_is_masked(dev, vector)) {
+            continue;
+        }
+        vq = virtio_get_queue(vdev, queue_no);
+        notifier = virtio_queue_get_guest_notifier(vq);
+        if (event_notifier_test_and_clear(notifier)) {
+            msix_set_pending(dev, vector);
+        }
+    }
+}
+
+static int virtio_pci_set_guest_notifier(DeviceState *d, int n, bool assign,
+                                         bool with_irqfd)
 {
     VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
     VirtQueue *vq = virtio_get_queue(proxy->vdev, n);
@@ -592,9 +617,9 @@ static int virtio_pci_set_guest_notifier(DeviceState *d, int n, bool assign)
         if (r < 0) {
             return r;
         }
-        virtio_queue_set_guest_notifier_fd_handler(vq, true, false);
+        virtio_queue_set_guest_notifier_fd_handler(vq, true, with_irqfd);
     } else {
-        virtio_queue_set_guest_notifier_fd_handler(vq, false, false);
+        virtio_queue_set_guest_notifier_fd_handler(vq, false, with_irqfd);
         event_notifier_cleanup(notifier);
     }
 
@@ -612,9 +637,11 @@ static int virtio_pci_set_guest_notifiers(DeviceState *d, bool assign)
     VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
     VirtIODevice *vdev = proxy->vdev;
     int r, n;
+    bool with_irqfd = msix_enabled(&proxy->pci_dev) &&
+        kvm_msi_via_irqfd_enabled();
 
     /* Must unset vector notifier while guest notifier is still assigned */
-    if (kvm_msi_via_irqfd_enabled() && !assign) {
+    if (with_irqfd && !assign) {
         msix_unset_vector_notifiers(&proxy->pci_dev);
         g_free(proxy->vector_irqfd);
         proxy->vector_irqfd = NULL;
@@ -625,21 +652,22 @@ static int virtio_pci_set_guest_notifiers(DeviceState *d, bool assign)
             break;
         }
 
-        r = virtio_pci_set_guest_notifier(d, n, assign);
+        r = virtio_pci_set_guest_notifier(d, n, assign,
+                                          kvm_msi_via_irqfd_enabled());
         if (r < 0) {
             goto assign_error;
         }
     }
 
     /* Must set vector notifier after guest notifier has been assigned */
-    if (kvm_msi_via_irqfd_enabled() && assign) {
+    if (with_irqfd && assign) {
         proxy->vector_irqfd =
             g_malloc0(sizeof(*proxy->vector_irqfd) *
                       msix_nr_vectors_allocated(&proxy->pci_dev));
         r = msix_set_vector_notifiers(&proxy->pci_dev,
                                       kvm_virtio_pci_vector_use,
                                       kvm_virtio_pci_vector_release,
-                                      NULL);
+                                      kvm_virtio_pci_vector_poll);
         if (r < 0) {
             goto assign_error;
         }
@@ -651,7 +679,7 @@ assign_error:
     /* We get here on assignment failure. Recover by undoing for VQs 0 .. n. */
     assert(assign);
     while (--n >= 0) {
-        virtio_pci_set_guest_notifier(d, n, !assign);
+        virtio_pci_set_guest_notifier(d, n, !assign, with_irqfd);
     }
     return r;
 }
-- 
MST