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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 29, 2016 at 10:05:22PM +0200, Maxime Coquelin wrote:
> 
> 
> 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.

In linux guest skbs are initialized AFAIK. See virtio_net_hdr_from_skb
first thing it does is
        memset(hdr, 0, sizeof(*hdr));



> > 
> > 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.

Yes but two points.

1. why is this memset expensive? Is the test completely skipping looking
   at the packet otherwise?

2. As long as we are doing this, see
	Alignment vs. Networking
	========================
in Documentation/unaligned-memory-access.txt


> 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