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authorStefan Hajnoczi <stefanha@linux.vnet.ibm.com>2010-12-17 12:01:50 +0000
committerMichael S. Tsirkin <mst@redhat.com>2011-01-10 14:44:16 +0200
commit25db9ebe15125deb32958c6df74996f745edf1f9 (patch)
treef1a5117bf6ffe11264f9da55b00ac5f803076269 /hw/virtio.h
parentd2f2b8a740c82319f9eea51ebed50815fbc3da3e (diff)
virtio-pci: Use ioeventfd for virtqueue notify
Virtqueue notify is currently handled synchronously in userspace virtio. This prevents the vcpu from executing guest code while hardware emulation code handles the notify. On systems that support KVM, the ioeventfd mechanism can be used to make virtqueue notify a lightweight exit by deferring hardware emulation to the iothread and allowing the VM to continue execution. This model is similar to how vhost receives virtqueue notifies. The result of this change is improved performance for userspace virtio devices. Virtio-blk throughput increases especially for multithreaded scenarios and virtio-net transmit throughput increases substantially. Some virtio devices are known to have guest drivers which expect a notify to be processed synchronously and spin waiting for completion. For virtio-net, this also seems to interact with the guest stack in strange ways so that TCP throughput for small message sizes (~200bytes) is harmed. Only enable ioeventfd for virtio-blk for now. Care must be taken not to interfere with vhost-net, which uses host notifiers. If the set_host_notifier() API is used by a device virtio-pci will disable virtio-ioeventfd and let the device deal with host notifiers as it wishes. Finally, there used to be a limit of 6 KVM io bus devices inside the kernel. On such a kernel, don't use ioeventfd for virtqueue host notification since the limit is reached too easily. This ensures that existing vhost-net setups (which always use ioeventfd) have ioeventfds available so they can continue to work. After migration and on VM change state (running/paused) virtio-ioeventfd will enable/disable itself. * VIRTIO_CONFIG_S_DRIVER_OK -> enable virtio-ioeventfd * !VIRTIO_CONFIG_S_DRIVER_OK -> disable virtio-ioeventfd * virtio_pci_set_host_notifier() -> disable virtio-ioeventfd * vm_change_state(running=0) -> disable virtio-ioeventfd * vm_change_state(running=1) -> enable virtio-ioeventfd Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Diffstat (limited to 'hw/virtio.h')
-rw-r--r--hw/virtio.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/hw/virtio.h b/hw/virtio.h
index bd5274225e..d8546d5b30 100644
--- a/hw/virtio.h
+++ b/hw/virtio.h
@@ -222,5 +222,6 @@ void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx);
VirtQueue *virtio_get_queue(VirtIODevice *vdev, int n);
EventNotifier *virtio_queue_get_guest_notifier(VirtQueue *vq);
EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);
+void virtio_queue_notify_vq(VirtQueue *vq);
void virtio_irq(VirtQueue *vq);
#endif