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

[Stefano Garzarella]

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