Re: [dpdk-dev] [PATCH v7 4/8] vhost: rxtx: use queue id instead of constant ring index

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

On Thu, Oct 22, 2015 at 05:49:55PM +0800, Yuanhan Liu wrote:
> On Wed, Oct 21, 2015 at 05:26:18PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Oct 21, 2015 at 08:48:15PM +0800, Yuanhan Liu wrote:
> > > > Please note that for virtio devices, guest is supposed to
> > > > control the placement of incoming packets in RX queues.
> > > 
> > > I may not follow you.
> > > 
> > > Enqueuing packets to a RX queue is done at vhost lib, outside the
> > > guest, how could the guest take the control here?
> > > 
> > > 	--yliu
> > 
> > vhost should do what guest told it to.
> > 
> > See virtio spec:
> > 	5.1.6.5.5 Automatic receive steering in multiqueue mode
> 
> Spec says:
> 
>     After the driver transmitted a packet of a flow on transmitqX,
>     the device SHOULD cause incoming packets for that flow to be
>     steered to receiveqX.
> 
> 
> Michael, I still have no idea how vhost could know the flow even
> after discussion with Huawei. Could you be more specific about
> this? Say, how could guest know that? And how could guest tell
> vhost which RX is gonna to use?
> 
> Thanks.
> 
> 	--yliu

I don't really understand the question.

When guests transmits a packet, it makes a decision
about the flow to use, and maps that to a tx/rx pair of queues.

It sends packets out on the tx queue and expects device to
return packets from the same flow on the rx queue.

During transmit, device needs to figure out the flow
of packets as they are received from guest, and track
which flows go on which tx queue.
When it selects the rx queue, it has to use the same table.

There is currently no provision for controlling
steering for uni-directional
flows which are possible e.g. with UDP.

We might solve this in a future spec - for example, set a flag notifying
guest that steering information is missing for a given flow, for example
by setting a flag in a packet, or using the command queue, and have
guest send a dummy empty packet to set steering rule for this flow.


-- 
MST