On Wed, Apr 14, 2021 at 2:38 AM Stefano Garzarella <sgarz...@redhat.com> wrote: > > On Wed, Apr 14, 2021 at 03:20:07AM -0400, Michael S. Tsirkin wrote: > >On Wed, Apr 14, 2021 at 08:57:06AM +0200, Stefano Garzarella wrote: > >> On Tue, Apr 13, 2021 at 03:58:34PM -0400, Michael S. Tsirkin wrote: > >> > On Tue, Apr 13, 2021 at 04:03:51PM +0200, Stefano Garzarella wrote: > >> > > On Tue, Apr 13, 2021 at 09:50:45AM -0400, Michael S. Tsirkin wrote: > >> > > > On Tue, Apr 13, 2021 at 03:38:52PM +0200, Stefano Garzarella wrote: > >> > > > > On Tue, Apr 13, 2021 at 09:16:50AM -0400, Michael S. Tsirkin wrote: > >> > > > > > On Tue, Apr 13, 2021 at 02:58:53PM +0200, Stefano Garzarella > >> > > > > > wrote: > >> > > > > > > On Mon, Apr 12, 2021 at 03:42:23PM -0700, Jiang Wang . wrote: > >> > > > > > > > On Mon, Apr 12, 2021 at 7:21 AM Stefano Garzarella > >> > > > > > > > <sgarz...@redhat.com> wrote: > >> > > > > > > > > > >> > > > > > > > > On Mon, Apr 12, 2021 at 02:50:17PM +0100, Stefan Hajnoczi > >> > > > > > > > > wrote: > >> > > > > > > > > >On Thu, Apr 01, 2021 at 04:36:02AM +0000, jiang.wang > >> > > > > > > > > >wrote: > > [...] > > >> > > > > > > > > >> > >> > > > > > > > > >> +Datagram sockets provide connectionless unreliable > >> > > > > > > > > >> messages of > >> > > > > > > > > >> +a fixed maximum length. > >> > > > > > > > > > > >> > > > > > > > > >Plus unordered (?) and with message boundaries. In other > >> > > > > > > > > >words: > >> > > > > > > > > > > >> > > > > > > > > > Datagram sockets provide unordered, unreliable, > >> > > > > > > > > > connectionless message > >> > > > > > > > > > with message boundaries and a fixed maximum length. > >> > > > > > > > > > > >> > > > > > > > > >I didn't think of the fixed maximum length aspect before. > >> > > > > > > > > >I guess the > >> > > > > > > > > >intention is that the rx buffer size is the message size > >> > > > > > > > > >limit? That's > >> > > > > > > > > >different from UDP messages, which can be fragmented into > >> > > > > > > > > >multiple IP > >> > > > > > > > > >packets and can be larger than 64KiB: > >> > > > > > > > > >https://en.wikipedia.org/wiki/User_Datagram_Protocol#UDP_datagram_structure > >> > > > > > > > > > > >> > > > > > > > > >Is it possible to support large datagram messages in > >> > > > > > > > > >vsock? I'm a little > >> > > > > > > > > >concerned that applications that run successfully over > >> > > > > > > > > >UDP will not be > >> > > > > > > > > >portable if vsock has this limitation because it would > >> > > > > > > > > >impose extra > >> > > > > > > > > >message boundaries that the application protocol might > >> > > > > > > > > >not tolerate. > >> > > > > > > > > > >> > > > > > > > > Maybe we can reuse the same approach Arseny is using for > >> > > > > > > > > SEQPACKET. > >> > > > > > > > > Fragment the packets according to the buffers in the > >> > > > > > > > > virtqueue and set > >> > > > > > > > > the EOR flag to indicate the last packet in the message. > >> > > > > > > > > > >> > > > > > > > Agree. Another option is to use the ones for skb since we > >> > > > > > > > may need to > >> > > > > > > > use skbs for multiple transport support anyway. > >> > > > > > > > > >> > > > > > > > >> > > > > > > The important thing I think is to have a single flag in > >> > > > > > > virtio-vsock that > >> > > > > > > identifies pretty much the same thing: this is the last > >> > > > > > > fragment of a series > >> > > > > > > to rebuild a packet. > >> > > > > > > > >> > > > > > > We should reuse the same flag for DGRAM and SEQPACKET. > >> > > > > > > > >> > > > > > > Thanks, > >> > > > > > > Stefano > >> > > > > > > >> > > > > > Well DGRAM can drop data so I wonder whether it can work ... > >> > > > > > > >> > > > > > >> > > > > Yep, this is true, but the channel should not be losing packets, > >> > > > > so if the > >> > > > > receiver discards packets, it knows that it must then discard all > >> > > > > of them > >> > > > > until the EOR. > >> > > > > >> > > > That is not so easy - they can come mixed up from multiple sources. > >> > > > >> > > I think we can prevent mixing because virtuqueue is point to point and > >> > > its > >> > > use is not thread safe, so the access (in the same peer) is already > >> > > serialized. > >> > > In the end the packet would be fragmented only before copying it to the > >> > > virtuqueue. > >> > > > >> > > But maybe I missed something... > >> > > >> > Well I ask what's the point of fragmenting then. I assume it's so we > >> > can pass huge messages around so you can't keep locks ... > >> > > >> > >> Maybe I'm wrong, but isn't this similar to what we do in virtio-net with > >> mergeable buffers? > > > >The point of mergeable buffers is to use less memory: both for each > >packet and for a full receive vq. > > > >> Also in this case I think the fragmentation will happen only in the device, > >> since the driver can enqueue the entire buffer. > >> > >> Maybe we can reuse mergeable buffers for virtio-vsock if the EOR flag is > >> not > >> suitable. > > > >That sounds very reasonable. > > It should also allow us to save the header for each fragment. > > @Jiang Do you want to explore this? > I'm talking about VIRTIO_NET_F_MRG_RXBUF feature.
Sure. Will do. > > > >> IIUC in the vsock device the fragmentation for DGRAM will happen just > >> before > >> to queue it in the virtqueue, and the device can check how many buffers are > >> available in the queue and it can decide whether to queue them all up or > >> throw them away. > >> > > >> > > > Sure linux net core does this but with fragmentation added in, > >> > > > I start wondering whether you are beginning to reinvent the net stack > >> > > > ... > >> > > > >> > > No, I hope not :-), in the end our advantage is that we have a channel > >> > > that > >> > > doesn't lose packets, so I guess we can make assumptions that the > >> > > network > >> > > stack can't. > >> > > > >> > > Thanks, > >> > > Stefano > >> > > >> > I still don't know how will credit accounting work for datagram, > >> > but proposals I saw seem to actually lose packets ... > >> > > >> > >> I still don't know too, but I think it's not an issue in the RX side, > >> since if it doesn't have space, can drop all the fragment. > >> > >> Another option to avoid fragmentation could be to allocate 64K buffers for > >> the new DGRAM virtqueue. > > > >That's a lot of buffers ... > > Yep I see, and they would often be mostly unused... > > > > >> In this way we will have at most 64K packets, which is similar to > >> UDP/IP, > >> without extra work for the fragmentation. > > > >IIRC default MTU is 1280 not 64K ... > > I was thinking that UDP at most can support 64K messages that IP should > fragment according to MTU. > > Thanks, > Stefano > _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
