Hi Arseny,
On Thu, Feb 18, 2021 at 08:33:44AM +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, two new packet operations were added: first for start of record
and second to mark end of record(SEQ_BEGIN and SEQ_END later). Also,
both operations carries metadata - to maintain boundaries and payload
integrity. Metadata is introduced by adding special header with two
fields - message count and message length:
struct virtio_vsock_seq_hdr {
__le32 msg_cnt;
__le32 msg_len;
} __attribute__((packed));
This header is transmitted as payload of SEQ_BEGIN and SEQ_END
packets(buffer of second virtio descriptor in chain) in the same way as
data transmitted in RW packets. Payload was chosen as buffer for this
header to avoid touching first virtio buffer which carries header of
packet, because someone could check that size of this buffer is equal
to size of packet header. To send record, packet with start marker is
sent first(it's header contains length of record and counter), then
counter is incremented and all data is sent as usual 'RW' packets and
finally SEQ_END is sent(it also carries counter of message, which is
counter of SEQ_BEGIN + 1), also after sedning SEQ_END counter is
incremented again. On receiver's side, length of record is known from
packet with start record marker. To check that no packets were dropped
by transport, counters of two sequential SEQ_BEGIN and SEQ_END are
checked(counter of SEQ_END must be bigger that counter of SEQ_BEGIN by
1) and length of data between two markers is compared to length in
SEQ_BEGIN header.
Now as packets of one socket are not reordered neither on
vsock nor on vhost transport layers, such markers 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.
I reviewed the first part (af_vsock.c changes), tomorrow I'll review the
rest. That part looks great to me, only found a few minor issues.
In the meantime, however, I'm getting a doubt, especially with regard to
other transports besides virtio.
Should we hide the begin/end marker sending in the transport?
I mean, should the transport just provide a seqpacket_enqueue()
callbacl?
Inside it then the transport will send the markers. This is because some
transports might not need to send markers.
But thinking about it more, they could actually implement stubs for that
calls, if they don't need to send markers.
So I think for now it's fine since it allows us to reuse a lot of code,
unless someone has some objection.
Thanks,
Stefano
