Merge branch 'vsock-add-multi-transports-support'

Stefano Garzarella says:

====================
vsock: add multi-transports support

Most of the patches are reviewed by Dexuan, Stefan, and Jorgen.
The following patches need reviews:
- [11/15] vsock: add multi-transports support
- [12/15] vsock/vmci: register vmci_transport only when VMCI guest/host
          are active
- [15/15] vhost/vsock: refuse CID assigned to the guest->host transport

RFC: https://patchwork.ozlabs.org/cover/1168442/
v1: https://patchwork.ozlabs.org/cover/1181986/

v1 -> v2:
- Patch 11:
    + vmci_transport: sent reset when vsock_assign_transport() fails
      [Jorgen]
    + fixed loopback in the guests, checking if the remote_addr is the
      same of transport_g2h->get_local_cid()
    + virtio_transport_common: updated space available while creating
      the new child socket during a connection request
- Patch 12:
    + removed 'features' variable in vmci_transport_init() [Stefan]
    + added a flag to register only once the host [Jorgen]
- Added patch 15 to refuse CID assigned to the guest->host transport in
  the vhost_transport

This series adds the multi-transports support to vsock, following
this proposal: https://www.spinics.net/lists/netdev/msg575792.html

With the multi-transports support, we can use VSOCK with nested VMs
(using also different hypervisors) loading both guest->host and
host->guest transports at the same time.
Before this series, vmci_transport supported this behavior but only
using VMware hypervisor on L0, L1, etc.

The first 9 patches are cleanups and preparations, maybe some of
these can go regardless of this series.

Patch 10 changes the hvs_remote_addr_init(). setting the
VMADDR_CID_HOST as remote CID instead of VMADDR_CID_ANY to make
the choice of transport to be used work properly.

Patch 11 adds multi-transports support.

Patch 12 changes a little bit the vmci_transport and the vmci driver
to register the vmci_transport only when there are active host/guest.

Patch 13 prevents the transport modules unloading while sockets are
assigned to them.

Patch 14 fixes an issue in the bind() logic discoverable only with
the new multi-transport support.

Patch 15 refuses CID assigned to the guest->host transport in the
vhost_transport.

I've tested this series with nested KVM (vsock-transport [L0,L1],
virtio-transport[L1,L2]) and with VMware (L0) + KVM (L1)
(vmci-transport [L0,L1], vhost-transport [L1], virtio-transport[L2]).

Dexuan successfully tested the RFC series on HyperV with a Linux guest.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>