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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 09/29/2016 10:21 PM, Michael S. Tsirkin wrote:
> 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

This change will not have an impact on the IP header alignment,
as is offset in the mbuf will not change.

Regards,
Maxime