Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Thu, Sep 29, 2016 at 10:05:22PM +0200, Maxime Coquelin wrote:
> >so doing this unconditionally would be a spec violation, but if you see
> >value in this, we can add a feature bit.
> Right it would be a spec violation, so it should be done conditionally.
> If a feature bit is to be added, what about VIRTIO_NET_F_NO_TX_HEADER?
> It would imply VIRTIO_NET_F_CSUM not set, and no GSO features set.
> If negotiated, we wouldn't need to prepend a header.

If we could skip Tx header, I think we could also skip Rx header, in the
case when mrg-rx is aslo turned off?

> From the micro-benchmarks results, we can expect +10% compared to
> indirect descriptors, and + 5% compared to using 2 descs in the
> virtqueue.
> Also, it should have the same benefits as indirect descriptors for 0%
> pkt loss (as we can fill 2x more packets in the virtqueue).
> 
> What do you think?

I would vote for this. It should yield maximum performance for the case
that it's guaranteed that packet size will always fit in a typical MTU
(1500).

	--yliu