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 10:07:10PM +0800, Yuanhan Liu wrote:
> On Thu, Oct 22, 2015 at 02:32:31PM +0300, Michael S. Tsirkin wrote:
> > 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.
> 
> Thanks for the length explanation, Michael!
> 
> I guess the key is are we able to get the table inside vhost-user
> lib? And, are you looking for something like following?
> 
> 	static int rte_vhost_enqueue_burst(pkts)
> 	{
> 		for_each_pkts(pkt) {
> 			int rxq = get_rxq_from_table(pkt);
> 	
> 			queue_to_rxq(pkt, rxq);
> 		}
> 	}
> 
> BTW, there should be such implementation at some where, right?
> If so, would you please point it to me?

See tun_flow_update in drivers/net/tun.c in Linux.


> In the meanwhile, I will read more doc/code to try to understand
> it.
> 
> 	--yliu
> 
> > 
> > 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