[Qemu-devel] [PATCH v4 0/4] virtio: Use ioeventfd for virtqueue notify

[Stefan Hajnoczi]

The v4 version includes:
 * Simpler start/stop ioeventfd mechanism using bool ioeventfd_started state
 * Support for migration
 * Handle deassign race condition to avoid dropping a virtqueue kick
 * Add missing kvm_enabled() check to kvm_has_many_ioeventfds()
 * Documentation updates for qdev -device with ioeventfd=on|off

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.

Now that this code is in virtio-pci.c it is possible to explicitly enable
devices for which virtio-ioeventfd should be used.  Only virtio-blk and
virtio-net are enabled at this time.