[RFC,v2,0/5] virtio/vsock: introduce MSG_EOR flag for SEQPACKET

Message

Arseny Krasnov Aug. 10, 2021, 11:38 a.m. UTC

This patchset implements support of MSG_EOR bit for SEQPACKET
AF_VSOCK sockets over virtio transport.
	First we need to define 'messages' and 'records' like this:
Message is result of sending calls: 'write()', 'send()', 'sendmsg()'
etc. It has fixed maximum length, and it bounds are visible using
return from receive calls: 'read()', 'recv()', 'recvmsg()' etc.
Current implementation based on message definition above.
	Record has unlimited length, it consists of multiple messages,
and bounds of record are visible via MSG_EOR flag returned from
'recvmsg()' call. Sender passes MSG_EOR to sending system call and
receiver will see MSG_EOR when corresponding message will be processed.
	Idea of patchset comes from POSIX: it says that SEQPACKET
supports record boundaries which are visible for receiver using
MSG_EOR bit. So, it looks like MSG_EOR is enough thing for SEQPACKET
and we don't need to maintain boundaries of corresponding send -
receive system calls. But, for 'sendXXX()' and 'recXXX()' POSIX says,
that all these calls operates with messages, e.g. 'sendXXX()' sends
message, while 'recXXX()' reads messages and for SEQPACKET, 'recXXX()'
must read one entire message from socket, dropping all out of size
bytes. Thus, both message boundaries and MSG_EOR bit must be supported
to follow POSIX rules.
	To support MSG_EOR new bit was added along with existing
'VIRTIO_VSOCK_SEQ_EOR': 'VIRTIO_VSOCK_SEQ_EOM'(end-of-message) - now it
works in the same way as 'VIRTIO_VSOCK_SEQ_EOR'. But 'VIRTIO_VSOCK_SEQ_EOR'
is used to mark 'MSG_EOR' bit passed from userspace.
	This patchset includes simple test for MSG_EOR.

 Arseny Krasnov(5):
  virtio/vsock: add 'VIRTIO_VSOCK_SEQ_EOM' bit
  vhost/vsock: support MSG_EOR bit processing
  virito/vsock: support MSG_EOR bit processing
  af_vsock: rename variables in receive loop
  vsock_test: update message bounds test for MSG_EOR

 drivers/vhost/vsock.c                   | 22 +++++++++++++---------
 include/uapi/linux/virtio_vsock.h       |  3 ++-
 net/vmw_vsock/af_vsock.c                | 10 +++++-----
 net/vmw_vsock/virtio_transport_common.c | 23 +++++++++++++++--------
 tools/testing/vsock/vsock_test.c        |  8 +++++++-
 5 files changed, 42 insertions(+), 24 deletions(-)

 v1 -> v2:
 - 'VIRTIO_VSOCK_SEQ_EOR' is renamed to 'VIRTIO_VSOCK_SEQ_EOM', to
   support backward compatibility.
 - use bitmask of flags to restore in vhost.c, instead of separated
   bool variable for each flag.
 - test for EAGAIN removed, as logically it is not part of this
   patchset(will be sent separately).
 - cover letter updated(added part with POSIX description).

Signed-off-by: Arseny Krasnov <[e-mail]>