Re: Re: [RFC] vsock: add multiple transports support for dgram

[Jiang Wang .]

Hi Jorgen,

Thanks for the detailed explanation and I agree with you. For the bind list,
my  prototype is doing
something similar to that. I will double check it.

Hi Stefano,

I don't have other questions for now. Thanks.

Regards,

Jiang

On Tue, Apr 13, 2021 at 5:52 AM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
>
> On Tue, Apr 13, 2021 at 12:12:50PM +0000, Jorgen Hansen wrote:
> >
> >
> >On 12 Apr 2021, at 20:53, Jiang Wang . <jiang.wang@xxxxxxxxxxxxx<mailto:jiang.wang@xxxxxxxxxxxxx>> wrote:
> >
> >On Mon, Apr 12, 2021 at 7:04 AM Stefano Garzarella <sgarzare@xxxxxxxxxx<mailto:sgarzare@xxxxxxxxxx>> wrote:
> >
> >Hi Jiang,
> >thanks for re-starting the multi-transport support for dgram!
> >
> >No problem.
> >
> >On Wed, Apr 07, 2021 at 11:25:36AM -0700, Jiang Wang . wrote:
> >On Wed, Apr 7, 2021 at 2:51 AM Jorgen Hansen <jhansen@xxxxxxxxxx<mailto:jhansen@xxxxxxxxxx>> wrote:
> >
> >
> >On 6 Apr 2021, at 20:31, Jiang Wang <jiang.wang@xxxxxxxxxxxxx<mailto:jiang.wang@xxxxxxxxxxxxx>> wrote:
> >
> >From: "jiang.wang<http://jiang.wang>" <jiang.wang@xxxxxxxxxxxxx<mailto:jiang.wang@xxxxxxxxxxxxx>>
> >
> >Currently, only VMCI supports dgram sockets. To supported
> >nested VM use case, this patch removes transport_dgram and
> >uses transport_g2h and transport_h2g for dgram too.
> >
> >I agree on this part, I think that's the direction to go.
> >transport_dgram was added as a shortcut.
> >
> >Got it.
> >
> >
> >Could you provide some background for introducing this change - are you
> >looking at introducing datagrams for a different transport? VMCI datagrams
> >already support the nested use case,
> >
> >Yes, I am trying to introduce datagram for virtio transport. I wrote a
> >spec patch for
> >virtio dgram support and also a code patch, but the code patch is still WIP.
> >When I wrote this commit message, I was thinking nested VM is the same as
> >multiple transport support. But now, I realize they are different.
> >Nested VMs may use
> >the same virtualization layer(KVM on KVM), or different virtualization layers
> >(KVM on ESXi). Thanks for letting me know that VMCI already supported nested
> >use cases. I think you mean VMCI on VMCI, right?
> >
> >but if we need to support multiple datagram
> >transports we need to rework how we administer port assignment for datagrams.
> >One specific issue is that the vmci transport won’t receive any datagrams for a
> >port unless the datagram socket has already been assigned the vmci transport
> >and the port bound to the underlying VMCI device (see below for more details).
> >
> >I see.
> >
> >The transport is assgined when sending every packet and
> >receiving every packet on dgram sockets.
> >
> >Is the intent that the same datagram socket can be used for sending packets both
> >In the host to guest, and the guest to directions?
> >
> >Nope. One datagram socket will only send packets to one direction, either to the
> >host or to the guest. My above description is wrong. When sending packets, the
> >transport is assigned with the first packet (with auto_bind).
> >
> >I'm not sure this is right.
> >The auto_bind on the first packet should only assign a local port to the
> >socket, but does not affect the transport to be used.
> >
> >A user could send one packet to the nested guest and another to the host
> >using the same socket, or am I wrong?
> >
> >OK. I think you are right.
> >
> >
> >The problem is when receiving packets. The listener can bind to the
> >VMADDR_CID_ANY
> >address. Then it is unclear which transport we should use. For stream
> >sockets, there will be a new socket for each connection, and transport
> >can be decided
> >at that time. For datagram sockets, I am not sure how to handle that.
> >
> >yes, this I think is the main problem, but maybe the sender one is even
> >more complicated.
> >
> >Maybe we should remove the 1:1 association we have now between vsk and
> >transport.
> >
> >Yes, I thought about that too. One idea is to define two transports in vsk.
> >For sending pkt, we can pick the right transport when we get the packet, like
> >in virtio_transport_send_pkt_info(). For receiving pkts, we have to check
> >and call both
> >transports dequeue callbacks if the local cid is CID_ANY.
> >
> >At least for DGRAM, for connected sockets I think the association makes
> >sense.
> >
> >Yeah. For a connected socket, we will only use one transport.
> >
> >For VMCI, does the same transport can be used for both receiving from
> >host and from
> >the guest?
> >
> >Yes, they're registered at different times, but it's the same transport.
> >
> >
> >For virtio, the h2g and g2h transports are different,, so we have to
> >choose
> >one of them. My original thought is to wait until the first packet
> >arrives.
> >
> >Another idea is that we always bind to host addr and use h2g
> >transport because I think that might
> >be more common. If a listener wants to recv packets from the host, then
> >it
> >should bind to the guest addr instead of CID_ANY.
> >
> >Yes, I remember we discussed this idea, this would simplify the
> >receiving, but there is still the issue of a user wanting to receive
> >packets from both the nested guest and the host.
> >
> >OK. Agree.
> >
> >Any other suggestions?
> >
> >
> >I think one solution could be to remove the 1:1 association between
> >DGRAM socket and transport.
> >
> >IIUC VMCI creates a skb for each received packet and queues it through
> >sk_receive_skb() directly in the struct sock.
> >
> >Then the .dgram_dequeue() callback dequeues them using
> >skb_recv_datagram().
> >
> >We can move these parts in the vsock core, and create some helpers to
> >allow the transports to enqueue received DGRAM packets in the same way
> >(and with the same format) directly in the struct sock.
> >
> >
> >I agree to use skbs (and move them to vscok core). We have another use case
> >which will need to use skb. But I am not sure how this helps with multiple
> >transport cases. Each transport has a dgram_dequeue callback. So we still
> >need to let vsk have multiple transports somehow. Could you elaborate how
> >using skb help with multiple transport support? Will that be similar to what I
> >mentioned above? Thanks.
> >
> >Moving away from the 1:1 association between DGRAM socket and transports sounds
> >like the right approach to me. A dgram socket bound to CID_ANY would be able to
> >use either h2g or g2h on a per dgram basis. If the socket is bound to a specific CID -
> >either host or the guest CID, it should only use either the h2g for host CID or g2h
> >for the guest CID. This would match the logic for the stream sockets.
> >
> >I like the idea of removing the dgram_dequeue callback from the transports and instead
> >having a call that allow the transports to enqueue received dgrams into the socket
> >receive queue as skbs. This is what the VMCI transport does today. Then the
> >vsock_dgram_recvmsg function will provide functionality similar to what
> >vmci_transport_dgram_dequeue does today. The current datagram format used was
> >created specifically for VMCI datagrams, but the header just contains source and dest
> >CID and port, so we should be able to use it as is.
> >
> >For sends from CID_ANY, the same logic as for streams in vsock_assign_transport can
> >be applied on each send - but without locking the dgram socket to a specific transport.
> >
> >So the above is mostly restating what Stefano proposed, so this was a verbose way
> >of agreeing with that.
>
> Jorgen, thank you very much!
> This is exactly what I had in mind, explained much better :-)
>
> We should look at the datagram header better because virtio-vsock uses
> 64 bits for CID and port, but I don't think it's a big problem.
>
> @Jiang, I think Jorgen answered you questions, but feel free to ask more
> if it's not clear.
>
> >
> >With respect to binding a dgram socket to a port, we could introduce a bound list for
> >dgram sockets just like we have for streams. However, for VMCI, the port space
> >is shared with other VMCI datagram clients (at the VMCI device level), so if a
> >dgram socket can potentially use the vmci transport, it should reserve the port
> >with the VMCI transport before assigning it to the socket. So similar to how
> >__vsock_bind_stream checks if an port is already bound/in use, the dgram socket
> >would have an additional call to potential transports to reserve the port. If the
> >port cannot be reserved with the transport, move on to the next port in the case
> >of VMADDR_PORT_ANY, or return EADDRINUSE otherwise. Once reserved,
> >It will ensure that VMCI can deliver datagrams to the specified port. A reserved
> >port should be released when the socket is removed from the bound list.
>
> Yes, I agree, it seems the right way to go.
>
> Thanks,
> Stefano
>