Re: [RFC PATCH v5 00/19] virtio/vsock: introduce SOCK_SEQPACKET support

[Stefano Garzarella]

On Wed, Feb 24, 2021 at 11:28:50AM +0300, Arseny Krasnov wrote:
> On 24.02.2021 11:23, Stefano Garzarella wrote:
> > On Wed, Feb 24, 2021 at 07:29:25AM +0300, Arseny Krasnov wrote:
> > > On 23.02.2021 17:50, Stefano Garzarella wrote:
> > > > On Mon, Feb 22, 2021 at 03:23:11PM +0100, Stefano Garzarella wrote:
> > > > > Hi Arseny,
> > > > >
> > > > > On Thu, Feb 18, 2021 at 08:33:44AM +0300, Arseny Krasnov wrote:
> > > > > > 	This patchset impelements support of SOCK_SEQPACKET for virtio
> > > > > > transport.
> > > > > > 	As SOCK_SEQPACKET guarantees to save record boundaries, so to
> > > > > > do it, two new packet operations were added: first for start of record
> > > > > > and second to mark end of record(SEQ_BEGIN and SEQ_END later). Also,
> > > > > > both operations carries metadata - to maintain boundaries and payload
> > > > > > integrity. Metadata is introduced by adding special header with two
> > > > > > fields - message count and message length:
> > > > > >
> > > > > > 	struct virtio_vsock_seq_hdr {
> > > > > > 		__le32  msg_cnt;
> > > > > > 		__le32  msg_len;
> > > > > > 	} __attribute__((packed));
> > > > > >
> > > > > > 	This header is transmitted as payload of SEQ_BEGIN and SEQ_END
> > > > > > packets(buffer of second virtio descriptor in chain) in the same way as
> > > > > > data transmitted in RW packets. Payload was chosen as buffer for this
> > > > > > header to avoid touching first virtio buffer which carries header of
> > > > > > packet, because someone could check that size of this buffer is equal
> > > > > > to size of packet header. To send record, packet with start marker is
> > > > > > sent first(it's header contains length of record and counter), then
> > > > > > counter is incremented and all data is sent as usual 'RW' packets and
> > > > > > finally SEQ_END is sent(it also carries counter of message, which is
> > > > > > counter of SEQ_BEGIN + 1), also after sedning SEQ_END counter is
> > > > > > incremented again. On receiver's side, length of record is known from
> > > > > > packet with start record marker. To check that no packets were dropped
> > > > > > by transport, counters of two sequential SEQ_BEGIN and SEQ_END are
> > > > > > checked(counter of SEQ_END must be bigger that counter of SEQ_BEGIN by
> > > > > > 1) and length of data between two markers is compared to length in
> > > > > > SEQ_BEGIN header.
> > > > > > 	Now as  packets of one socket are not reordered neither on
> > > > > > vsock nor on vhost transport layers, such markers allows to restore
> > > > > > original record on receiver's side. If user's buffer is smaller that
> > > > > > record length, when all out of size data is dropped.
> > > > > > 	Maximum length of datagram is not limited as in stream socket,
> > > > > > because same credit logic is used. Difference with stream socket is
> > > > > > that user is not woken up until whole record is received or error
> > > > > > occurred. Implementation also supports 'MSG_EOR' and 'MSG_TRUNC' flags.
> > > > > > 	Tests also implemented.
> > > > > I reviewed the first part (af_vsock.c changes), tomorrow I'll review
> > > > > the rest. That part looks great to me, only found a few minor issues.
> > > > I revieiwed the rest of it as well, left a few minor comments, but I
> > > > think we're well on track.
> > > >
> > > > I'll take a better look at the specification patch tomorrow.
> > > Great, Thank You
> > > > Thanks,
> > > > Stefano
> > > >
> > > > > In the meantime, however, I'm getting a doubt, especially with regard
> > > > > to other transports besides virtio.
> > > > >
> > > > > Should we hide the begin/end marker sending in the transport?
> > > > >
> > > > > I mean, should the transport just provide a seqpacket_enqueue()
> > > > > callbacl?
> > > > > Inside it then the transport will send the markers. This is because
> > > > > some transports might not need to send markers.
> > > > >
> > > > > But thinking about it more, they could actually implement stubs for
> > > > > that calls, if they don't need to send markers.
> > > > >
> > > > > So I think for now it's fine since it allows us to reuse a lot of
> > > > > code, unless someone has some objection.
> > > I thought about that, I'll try to implement it in next version. Let's see...
> > If you want to discuss it first, write down the idea you want to
> > implement, I wouldn't want to make you do unnecessary work. :-)
>
> Idea is simple, in iov iterator of 'struct msghdr' which is passed to
>
> enqueue callback we have two fields: 'iov_offset' which is byte
>
> offset inside io vector where next data must be picked and 'count'
>
> which is rest of unprocessed bytes in io vector. So in seqpacket
>
> enqueue callback if 'iov_offset' is 0 i'll send SEQBEGIN, and if
>
> 'count' is 0 i'll send SEQEND.

Got it, make sense and it's defently more transparent for the vsock 
core!
Go head, maybe adding a comment in the vsock core explaining this, so 
other developers can understand better if they want to support SEPACKET 
in other transports.

Thanks,
Stefano