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 > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
