On Mon, Feb 28, 2011 at 11:12 AM, Amit Shah <amit.s...@redhat.com> wrote: > Enable ioeventfd for virtio-serial devices by default. Commit > 25db9ebe15125deb32958c6df74996f745edf1f9 lists the benefits of using > ioeventfd. > > Copying a file from guest to host over a virtio-serial channel didn't > show much difference in time or io_exit rate.
The cost of enabling ioeventfd is one eventfd file descriptor and KVM in-kernel device slot per virtqueue. The current maximum number per VM is 200, this is a kernel limit in include/linux/kvm_host.h:NR_IOBUS_DEVS. Do you really want to use ioeventfd for virtio-serial? Perhaps this is more useful for high-frequency device interfaces. Stefan
