Re: [RFC v2] virtio-vsock: add description for datagram type

[Stefano Garzarella <sgarzare@redhat.com>]

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 <sgarzare@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.

>
>> 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

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