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

[Stefano Garzarella]

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?

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.

> - 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