Re: [Qemu-devel] Make virtio-net.c ring size configurable?

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Feb 14, 2014 at 02:43:14PM +0100, Luke Gorrie wrote:
> Observation: virtio-net.c hard-codes the vring size to 256 buffers.
> 
> Could this reasonably be made configurable, or would that be likely to
> cause a problem?
> 
> In Snabb Switch we are creating a 1:1 mapping between Virtio-net
> descriptors and VMDq hardware receive descriptors. The VMDq queues support
> 32768 buffers and I'd like to match this on the QEMU/Virtio-net side -- or
> at least come close.

In reality virtio-net can use many more buffers because it has the
VIRTIO_RING_F_INDIRECT_DESC feature.  Each descriptor can point to a
whole new descriptor table.

Do you want the 1:1 mapping to achieve best performance or just to
simplify the coding?

Since vhost_net does many Gbit/s I doubt the ring size is a limiting
factor although there are still periodic discussions about tweaking the
direct vs indirect descriptor heuristic.

Stefan