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

Stefano Garzarella
2021-02-24T08:25:01Z

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. :-)

Cheers,
Stefano