[Qemu-devel] [PATCH V2] virtio-net: calculate proper msix vectors on init

[Jason Wang <jasowang@redhat.com>]

Currently, the default msix vectors for virtio-net-pci is 3 which is
obvious not suitable for multiqueue guest, so we depends on the user
or management tools to pass a correct vectors parameter. In fact, we
can simplifying this by calculate the number of vectors on init.

Consider we have N queues, the number of vectors needed is 2*N + 2
(plus one config interrupt and control vq). We didn't check whether or
not host support control vq because it was added unconditionally by
qemu to avoid breaking legacy guests such as Minix.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 hw/virtio/virtio-pci.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 7b91841..3b3b0e2 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -1416,7 +1416,8 @@ static const TypeInfo virtio_serial_pci_info = {
 static Property virtio_net_properties[] = {
     DEFINE_PROP_BIT("ioeventfd", VirtIOPCIProxy, flags,
                     VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT, false),
-    DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, 3),
+    DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors,
+                       DEV_NVECTORS_UNSPECIFIED),
     DEFINE_VIRTIO_NET_FEATURES(VirtIOPCIProxy, host_features),
     DEFINE_NIC_PROPERTIES(VirtIONetPCI, vdev.nic_conf),
     DEFINE_VIRTIO_NET_PROPERTIES(VirtIONetPCI, vdev.net_conf),
@@ -1428,6 +1429,11 @@ static int virtio_net_pci_init(VirtIOPCIProxy *vpci_dev)
     DeviceState *qdev = DEVICE(vpci_dev);
     VirtIONetPCI *dev = VIRTIO_NET_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
+    VirtIONet *net = VIRTIO_NET(&dev->vdev);
+
+    if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
+        vpci_dev->nvectors = 2 * MAX(net->nic_conf.queues, 1) + 2;
+    }
 
     virtio_net_set_config_size(&dev->vdev, vpci_dev->host_features);
     virtio_net_set_netclient_name(&dev->vdev, qdev->id,
-- 
1.8.3.2