Merge branch 'vsock-updates-for-so_rcvlowat-handling'

Merge branch 'vsock-updates-for-so_rcvlowat-handling'

Arseniy Krasnov says:

====================
vsock: updates for SO_RCVLOWAT handling

This patchset includes some updates for SO_RCVLOWAT:

1) af_vsock:
   During my experiments with zerocopy receive, i found, that in some
   cases, poll() implementation violates POSIX: when socket has non-
   default SO_RCVLOWAT(e.g. not 1), poll() will always set POLLIN and
   POLLRDNORM bits in 'revents' even number of bytes available to read
   on socket is smaller than SO_RCVLOWAT value. In this case,user sees
   POLLIN flag and then tries to read data(for example using  'read()'
   call), but read call will be blocked, because  SO_RCVLOWAT logic is
   supported in dequeue loop in af_vsock.c. But the same time,  POSIX
   requires that:

   "POLLIN     Data other than high-priority data may be read without
               blocking.
    POLLRDNORM Normal data may be read without blocking."

   See https://www.open-std.org/jtc1/sc22/open/n4217.pdf, page 293.

   So, we have, that poll() syscall returns POLLIN, but read call will
   be blocked.

   Also in man page socket(7) i found that:

   "Since Linux 2.6.28, select(2), poll(2), and epoll(7) indicate a
   socket as readable only if at least SO_RCVLOWAT bytes are available."

   I checked TCP callback for poll()(net/ipv4/tcp.c, tcp_poll()), it
   uses SO_RCVLOWAT value to set POLLIN bit, also i've tested TCP with
   this case for TCP socket, it works as POSIX required.

   I've added some fixes to af_vsock.c and virtio_transport_common.c,
   test is also implemented.

2) virtio/vsock:
   It adds some optimization to wake ups, when new data arrived. Now,
   SO_RCVLOWAT is considered before wake up sleepers who wait new data.
   There is no sense, to kick waiter, when number of available bytes
   in socket's queue < SO_RCVLOWAT, because if we wake up reader in
   this case, it will wait for SO_RCVLOWAT data anyway during dequeue,
   or in poll() case, POLLIN/POLLRDNORM bits won't be set, so such
   exit from poll() will be "spurious". This logic is also used in TCP
   sockets.

3) vmci/vsock:
   Same as 2), but i'm not sure about this changes. Will be very good,
   to get comments from someone who knows this code.

4) Hyper-V:
   As Dexuan Cui mentioned, for Hyper-V transport it is difficult to
   support SO_RCVLOWAT, so he suggested to disable this feature for
   Hyper-V.
====================

Link: https://lore.kernel.org/r/de41de4c-0345-34d7-7c36-4345258b7ba8@sberdevices.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>