AF_VSOCK loopback

[Stefan Hajnoczi <stefanha@redhat.com>]

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Hi Jorgen,
Cathy Avery found that the AF_VSOCK VMCI transport does loopback inside
the guest (but not on the host?).  The virtio transport currently does
no loopback.

The loopback scenario I'm thinking of is where process A listens on port
1234 and process B on the same machine connects to port 1234 both with
the same CID.

I'd like to make the virtio transport compatible with VMCI transport
semantics so AF_VSOCK behaves the same regardless of the transport.
This means loopback must be added to virtio-vsock.

The core net/vmware/af_vsock.c code does not implement loopback.  How
does VMCI do loopback?  Are the loopback packets reflected back from the
host?  Or does the guest driver notice the loopback and avoid passing
packets to the host in the first place?

Maybe we can make the loopback code common in af_vsock.c if that avoids
code duplication.

Thanks,
Stefan

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