Re: [RFC PATCH v3 00/13] virtio/vsock: introduce SOCK_SEQPACKET support

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Jan 29, 2021 at 09:41:50AM +0300, Arseny Krasnov wrote:
>
>On 28.01.2021 20:19, Stefano Garzarella wrote:
>> Hi Arseny,
>> I reviewed a part, tomorrow I hope to finish the other patches.
>>
>> Just a couple of comments in the TODOs below.
>>
>> On Mon, Jan 25, 2021 at 02:09:00PM +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, new packet operation was added: it marks start of record (with
>>> record length in header), such packet doesn't carry any data.  To send
>>> record, packet with start marker is sent first, then all data is sent
>>> as usual 'RW' packets. On receiver's side, length of record is known
>> >from packet with start record marker. Now as  packets of one socket
>>> are not reordered neither on vsock nor on vhost transport layers, such
>>> marker 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.
>>>
>>> Arseny Krasnov (13):
>>>  af_vsock: prepare for SOCK_SEQPACKET support
>>>  af_vsock: prepare 'vsock_connectible_recvmsg()'
>>>  af_vsock: implement SEQPACKET rx loop
>>>  af_vsock: implement send logic for SOCK_SEQPACKET
>>>  af_vsock: rest of SEQPACKET support
>>>  af_vsock: update comments for stream sockets
>>>  virtio/vsock: dequeue callback for SOCK_SEQPACKET
>>>  virtio/vsock: fetch length for SEQPACKET record
>>>  virtio/vsock: add SEQPACKET receive logic
>>>  virtio/vsock: rest of SOCK_SEQPACKET support
>>>  virtio/vsock: setup SEQPACKET ops for transport
>>>  vhost/vsock: setup SEQPACKET ops for transport
>>>  vsock_test: add SOCK_SEQPACKET tests
>>>
>>> drivers/vhost/vsock.c                   |   7 +-
>>> include/linux/virtio_vsock.h            |  12 +
>>> include/net/af_vsock.h                  |   6 +
>>> include/uapi/linux/virtio_vsock.h       |   9 +
>>> net/vmw_vsock/af_vsock.c                | 543 ++++++++++++++++------
>>> net/vmw_vsock/virtio_transport.c        |   4 +
>>> net/vmw_vsock/virtio_transport_common.c | 295 ++++++++++--
>>> tools/testing/vsock/util.c              |  32 +-
>>> tools/testing/vsock/util.h              |   3 +
>>> tools/testing/vsock/vsock_test.c        | 126 +++++
>>> 10 files changed, 862 insertions(+), 175 deletions(-)
>>>
>>> TODO:
>>> - Support for record integrity control. As transport could drop some
>>>   packets, something like "record-id" and record end marker need to
>>>   be implemented. Idea is that SEQ_BEGIN packet carries both record
>>>   length and record id, end marker(let it be SEQ_END) carries only
>>>   record id. To be sure that no one packet was lost, receiver checks
>>>   length of data between SEQ_BEGIN and SEQ_END(it must be same with
>>>   value in SEQ_BEGIN) and record ids of SEQ_BEGIN and SEQ_END(this
>>>   means that both markers were not dropped. I think that easiest way
>>>   to implement record id for SEQ_BEGIN is to reuse another field of
>>>   packet header(SEQ_BEGIN already uses 'flags' as record length).For
>>>   SEQ_END record id could be stored in 'flags'.
>> I don't really like the idea of reusing the 'flags' field for this
>> purpose.
>>
>>>     Another way to implement it, is to move metadata of both SEQ_END
>>>   and SEQ_BEGIN to payload. But this approach has problem, because
>>>   if we move something to payload, such payload is accounted by
>>>   credit logic, which fragments payload, while payload with record
>>>   length and id couldn't be fragmented. One way to overcome it is to
>>>   ignore credit update for SEQ_BEGIN/SEQ_END packet.Another solution
>>>   is to update 'stream_has_space()' function: current implementation
>>>   return non-zero when at least 1 byte is allowed to use,but updated
>>>   version will have extra argument, which is needed length. For 'RW'
>>>   packet this argument is 1, for SEQ_BEGIN it is sizeof(record len +
>>>   record id) and for SEQ_END it is sizeof(record id).
>> Is the payload accounted by credit logic also if hdr.op is not
>> VIRTIO_VSOCK_OP_RW?
>
>Yes, on send any packet with payload could be fragmented if
>
>there is not enough space at receiver. On receive 'fwd_cnt' and
>
>'buf_alloc' are updated with header of every packet. Of course,
>
>to every such case i've described i can add check for 'RW'
>
>packet, to exclude payload from credit accounting, but this is
>
>bunch of dumb checks.
>
>>
>> I think that we can define a specific header to put after the
>> virtio_vsock_hdr when hdr.op is SEQ_BEGIN or SEQ_END, and in this header
>> we can store the id and the length of the message.
>
>I think it is better than use payload and touch credit logic
>

Cool, so let's try this option, hoping there aren't a lot of issues.

Another item for TODO could be to add the SOCK_SEQPACKET support also 
for vsock_loopback. Should be simple since it also uses 
virtio_transport_common APIs and it can be useful for testing and 
debugging.

Thanks,
Stefano

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