Date:	Wed, 4 Feb 2015 05:51:47 +0000
From:	"Zayats, Michael" <michael.zayats@...com>
To:	John Fastabend <john.fastabend@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	dborkman <dborkman@...hat.com>
Subject: RE: AF_NETDEV - device specific sockets

> > More specific example would be when NIC performs certain fast path
> > processing, while punting to the CPU for a slow path.
> > Slow path would be interested to know the punt reason.
> >
> > Another example would be if specific NIC strips S-tag in QinQ case and
> > would like to communicate the stripped Tag to the client.
> >
> 
> Right, maybe we need some sort of TLV scheme to pass up the relevant
> info. I'm not sure we want to necessarily bury it in the driver though.
> Perhaps passing auxdata in a TLV format is worth considering.

CMSG formatting in sockets msg_control is pretty close, right?

> 
> Just curious do you have NICs that are stripping or inserting more then
> a single tag?

No, just a single tag.

> 
> For tagging my current scheme is to strip outer tags using this
> experimental Flow  API
> 
> 	http://www.spinics.net/lists/netdev/msg313071.html
> 
> and then only report the inner tag to the stack. At the moment I haven't
> found any use cases this is not sufficient.
> 
> > There might be many types of custom functionality, agreed between the
> > NIC and the clients, which is not generic or not practical enough for
> inclusion in the kernel.
> >
> > That's why I am looking for a generic, socket like mechanism of
> > device<->client, packet + metadata communication, which wouldn't
> require core kernel modification.
> 
> hmm the question is how do the NIC and client "agree" on the format of
> the data and its meaning? If you follow the thread above and also our
> af_packet direct DMA work we are struggling with similar questions,
> 
> 	http://www.spinics.net/lists/netdev/msg311862.html
> 
> I think we need some way to "describe" the meta-data or we need to build
> some kernel/uapi standard that defines them.
> 

Agreed, some kind of standard way to describe the language is needed.
However, I don't think it should preclude us from having a generic mechanism for communicating custom metadata alongside the packets. When client knows, which NIC type it "talks to".
So far, all the custom drivers that I have seen that needed it, ended up exposing chr device and interposing a custom header between the packets. Same approach that TUN is taking for virtio.

> .John
> 
> >
> > Thanks,
> >
> > Michael
> >
> >
> >
> >
> >
> > -----Original Message-----
> > From: John Fastabend [mailto:john.fastabend@...il.com]
> > Sent: Saturday, January 31, 2015 8:41 PM
> > To: Zayats, Michael
> > Cc: netdev@...r.kernel.org
> > Subject: Re: AF_NETDEV - device specific sockets
> >
> > On 01/31/2015 08:20 PM, Zayats, Michael wrote:
> >> Hi,
> >>
> >> I am looking for a generic mechanism that would allow network device
> >> drivers to provide socket interface to user and kernel space clients.
> >>
> >> Such an interface might be used to provide access to important
> >> sub-streams of packets, alongside with device specific packet
> >> metadata, provided through msg_control fields of recv/sendmsg.
> >>
> >> RX Metadata might include device specific information, such as
> >> queuing priorities applied, potential destination interface in case
> >> of switching hardware etc.
> >>
> >> On the transmission, metadata might be used to indicate hardware
> >> specific required optimizations, as well as any other transformation
> >> or accounting required on the packet.
> >>
> >> AF_PACKET based mechanism doesn't allow metadata to be exchanged
> >> between the client and the device driver. Extending it would require
> >> extending of sk_buff and potentially additional per packet
> operations.
> >> Generic Netlink is not intended to pass packets.
> >>
> >> As I am trying to validate generic applicability of such a mechanism,
> >> I see that TUN driver is providing custom socket interface, in order
> >> to deal with user information through msg_control. Only usable inside
> >> the kernel, through custom interface.
> >
> >> Proposed interface
> >> ------------------
> >> Kernel side:
> >> (struct proto *) should be added to struct net_device.
> >> Device driver that is interested to support socket interface would
> populate the pointer.
> >>
> >
> >> User space: After creating AF_NETDEV socket, the only successful
> >> operation would be setting SO_BINDTODEVICE option. Once set, all
> >> socket operations would be implemented by calling functions, that are
> >> registered at struct proto on the appropriate net_device.
> >>
> >> What do you think?
> >> Would you see a better approach?
> >> Some other mechanism that already exists for such a purpose?
> >
> > It might help to come up with specific examples but an alternate
> proposal would be to use skb->priority field and then mqprio to steer
> the traffic to a specific queue and then bind attributes to the queue.
> >
> > For example the NIC offloaded QOS can be mapped on to queues and then
> sockets mapped to the queues.
> >
> > Another example would be to forward all traffic from one queue to a
> virtual fuction in SR-IOV use case. We don't have an interface to do
> this but I have been working on an API that could be used for this.
> >
> > In this case you don't need to modify AF_PACKET interface but
> configure the device correctly. If you need per-packet control you could
> use 'tc' or 'nftables' to do the steering.
> >
> > .John
> >
> 
> 
> --
> John Fastabend         Intel Corporation
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