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    SubjectRe: [RFC PATCH v5 00/19] virtio/vsock: introduce SOCK_SEQPACKET support
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    On 24.02.2021 11:35, Stefano Garzarella wrote:
    > 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.
    Ack
    >
    > Thanks,
    > Stefano
    >
    >

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