Message-ID: <20210806071643.byebg4hmm3dtnb2x@steredhat>
Date:   Fri, 6 Aug 2021 09:16:43 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseny Krasnov <arseny.krasnov@...persky.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Colin Ian King <colin.king@...onical.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Norbert Slusarek <nslusarek@....net>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "oxffffaa@...il.com" <oxffffaa@...il.com>
Subject: Re: [!!Mass Mail KSE][MASSMAIL KLMS] Re: [RFC PATCH v1 0/7]
 virtio/vsock: introduce MSG_EOR flag for SEQPACKET

On Thu, Aug 05, 2021 at 12:21:57PM +0300, Arseny Krasnov wrote:
>
>On 05.08.2021 12:06, Stefano Garzarella wrote:
>> Caution: This is an external email. Be cautious while opening links or attachments.
>>
>>
>>
>> On Thu, Aug 05, 2021 at 11:33:12AM +0300, Arseny Krasnov wrote:
>>> On 04.08.2021 15:57, Stefano Garzarella wrote:
>>>> Caution: This is an external email. Be cautious while opening links or attachments.
>>>>
>>>>
>>>>
>>>> Hi Arseny,
>>>>
>>>> On Mon, Jul 26, 2021 at 07:31:33PM +0300, Arseny Krasnov wrote:
>>>>>       This patchset implements support of MSG_EOR bit for SEQPACKET
>>>>> AF_VSOCK sockets over virtio transport.
>>>>>       Idea is to distinguish concepts of 'messages' and 'records'.
>>>>> Message is result of sending calls: 'write()', 'send()', 'sendmsg()'
>>>>> etc. It has fixed maximum length, and it bounds are visible using
>>>>> return from receive calls: 'read()', 'recv()', 'recvmsg()' etc.
>>>>> Current implementation based on message definition above.
>>>> Okay, so the implementation we merged is wrong right?
>>>> Should we disable the feature bit in stable kernels that contain it? Or
>>>> maybe we can backport the fixes...
>>> Hi,
>>>
>>> No, this is correct and it is message boundary based. Idea of this
>>> patchset is to add extra boundaries marker which i think could be
>>> useful when we want to send data in seqpacket mode which length
>>> is bigger than maximum message length(this is limited by transport).
>>> Of course we can fragment big piece of data too small messages, but
>>> this
>>> requires to carry fragmentation info in data protocol. So In this case
>>> when we want to maintain boundaries receiver calls recvmsg() until
>>> MSG_EOR found.
>>> But when receiver knows, that data is fit in maximum datagram length,
>>> it doesn't care about checking MSG_EOR just calling recv() or
>>> read()(e.g.
>>> message based mode).
>> I'm not sure we should maintain boundaries of multiple send(), from
>> POSIX standard [1]:
>
>Yes, but also from POSIX: such calls like send() and sendmsg()
>
>operates with "message" and if we check recvmsg() we will
>
>find the following thing:
>
>
>For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire
>
>message shall be read in a single operation. If a message is too long to fit in the supplied
>
>buffers, and MSG_PEEK is not set in the flags argument, the excess bytes shall be discarded.
>
>
>I understand this, that send() boundaries also must be maintained.
>
>I've checked SEQPACKET in AF_UNIX and AX_25 - both doesn't support
>
>MSG_EOR, so send() boundaries must be supported.
>
>>
>>    SOCK_SEQPACKET
>>      Provides sequenced, reliable, bidirectional, connection-mode
>>      transmission paths for records. A record can be sent using one or
>>      more output operations and received using one or more input
>>      operations, but a single operation never transfers part of more than
>>      one record. Record boundaries are visible to the receiver via the
>>      MSG_EOR flag.
>>
>>  From my understanding a record could be sent with multiple send() 
>>  and
>> received, for example, with a single recvmsg().
>> The only boundary should be the MSG_EOR flag set by the user on the 
>> last
>> send() of a record.
>You are right, if we talking about "record".
>>
>>  From send() description [2]:
>>
>>    MSG_EOR
>>      Terminates a record (if supported by the protocol).
>>
>>  From recvmsg() description [3]:
>>
>>    MSG_EOR
>>      End-of-record was received (if supported by the protocol).
>>
>> Thanks,
>> Stefano
>>
>> [1]
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/socket.html
>> [2] 
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/send.html
>> [3]
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/recvmsg.html
>
>P.S.: seems SEQPACKET is too exotic thing that everyone implements it 
>in
>
>own manner, because i've tested SCTP seqpacket implementation, and 
>found
>
>that:
>
>1) It doesn't support MSG_EOR bit at send side, but uses MSG_EOR at 
>receiver
>
>side to mark MESSAGE boundary.
>
>2) According POSIX any extra bytes that didn't fit in user's buffer 
>must be dropped,
>
>but SCTP doesn't drop it - you can read rest of datagram in next calls.
>

Thanks for this useful information, now I see the differences and why we 
should support both.

I think is better to include them in the cover letter.

I'm going to review the paches right now :-)

Stefano

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