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

[Maxime Coquelin]

On 09/29/2016 07:57 PM, Michael S. Tsirkin wrote:
> On Thu, Sep 29, 2016 at 05:30:53PM +0200, Maxime Coquelin wrote:
...
> > Before enabling anything by default, we should first optimize the 1 slot
> > case. Indeed, micro-benchmark using testpmd in txonly[0] shows ~17%
> > perf regression for 64 bytes case:
> >  - 2 descs per packet: 11.6Mpps
> >  - 1 desc per packet: 9.6Mpps
> >
> > This is due to the virtio header clearing in virtqueue_enqueue_xmit().
> > Removing it, we get better results than with 2 descs (1.20Mpps).
> > Since the Virtio PMD doesn't support offloads, I wonder whether we can
> > just drop the memset?
>
> What will happen? Will the header be uninitialized?
Yes..
I didn't look closely at the spec, but just looked at DPDK's and Linux
vhost implementations. IIUC, the header is just skipped in the two
implementations.
> The spec says:
>         The driver can send a completely checksummed packet. In this case, flags
>         will be zero, and gso_type
>         will be VIRTIO_NET_HDR_GSO_NONE.
>
> and
>         The driver MUST set num_buffers to zero.
>         If VIRTIO_NET_F_CSUM is not negotiated, the driver MUST set flags to
>         zero and SHOULD supply a fully
>         checksummed packet to the device.
>
> and
>         If none of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have been
>         negotiated, the driver MUST
>         set gso_type to VIRTIO_NET_HDR_GSO_NONE.
>
> 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.

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?

Thanks,
Maxime