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

[Stefano Garzarella <sgarzare@redhat.com>]

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.

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.
>
>Thanks,
>Stefano
>