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Subject: Re: [RFC PATCH v5 00/19] virtio/vsock: introduce SOCK_SEQPACKET
 support
To:     Stefano Garzarella <sgarzare@redhat.com>
CC:     Stefan Hajnoczi <stefanha@redhat.com>,
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From:   Arseny Krasnov <arseny.krasnov@kaspersky.com>
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Date:   Wed, 24 Feb 2021 11:36:14 +0300
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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
>
>