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

[Arseny Krasnov]

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

>
>> - What to do, when server doesn't support SOCK_SEQPACKET. In current
>>   implementation RST is replied in the same way when listening port
>>   is not found. I think that current RST is enough,because case when
>>   server doesn't support SEQ_PACKET is same when listener missed(e.g.
>>   no listener in both cases).
> I think so, but I'll check better if we can have some issues.
>
> Thanks,
> Stefano
>
>> v2 -> v3:
>> - patches reorganized: split for prepare and implementation patches
>> - local variables are declared in "Reverse Christmas tree" manner
>> - virtio_transport_common.c: valid leXX_to_cpu() for vsock header
>>   fields access
>> - af_vsock.c: 'vsock_connectible_*sockopt()' added as shared code
>>   between stream and seqpacket sockets.
>> - af_vsock.c: loops in '__vsock_*_recvmsg()' refactored.
>> - af_vsock.c: 'vsock_wait_data()' refactored.
>>
>> v1 -> v2:
>> - patches reordered: af_vsock.c related changes now before virtio vsock
>> - patches reorganized: more small patches, where +/- are not mixed
>> - tests for SOCK_SEQPACKET added
>> - all commit messages updated
>> - af_vsock.c: 'vsock_pre_recv_check()' inlined to
>>   'vsock_connectible_recvmsg()'
>> - af_vsock.c: 'vsock_assign_transport()' returns ENODEV if transport
>>   was not found
>> - virtio_transport_common.c: transport callback for seqpacket dequeue
>> - virtio_transport_common.c: simplified
>>   'virtio_transport_recv_connected()'
>> - virtio_transport_common.c: send reset on socket and packet type
>> 			      mismatch.
>>
>> Signed-off-by: Arseny Krasnov <arseny.krasnov@xxxxxxxxxxxxx>
>>
>> -- 
>> 2.25.1
>>
>