virtio_net / netmap RX dropping frames

[Joe Buehler]

I am running virtio_net (netmap-modified) on top of netmap (latest) in a
KVM virtual machine.  The host adapter is Intel 82599ES 10G and the VM
is connected to it via macvtap.

My test setup is a small program in the VM sending frames out to an
external loopback device and watching what comes back.

I am running at fairly low frame rates (200k frames / sec) and seeing RX
frame drops and high latency (a few milliseconds).  The TX frames are
all making it to the external loopback device (based on device counters)
but the macvtap device in the RX path is reporting dropped frames, the
count agreeing with what the test program observes.

I guess my first question has to do with ring sizes.  The netmap API is
reporting 255 buffers in the RX and TX rings.  How do I increase this
substantially?

Joe