vsock.7: Add VMADDR_CID_LOCAL description

Linux 5.6 added the new well-known VMADDR_CID_LOCAL for
local communication.

This patch explains how to use it and removes the legacy
VMADDR_CID_RESERVED no longer available.

Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/vsock.7

@@ -127,8 +127,8 @@ There are several special addresses:
 means any address for binding;
 .B VMADDR_CID_HYPERVISOR
 (0) is reserved for services built into the hypervisor;
-.B VMADDR_CID_RESERVED
-(1) must not be used;
+.B VMADDR_CID_LOCAL
+(1) is the well-known address for local communication (loopback);
 .B VMADDR_CID_HOST
 (2)
 is the well-known address of the host.
@@ -164,6 +164,15 @@ Consider using
 .B VMADDR_CID_ANY
 when binding instead of getting the local CID with
 .BR IOCTL_VM_SOCKETS_GET_LOCAL_CID .
+.SS Local communication
+.B VMADDR_CID_LOCAL
+(1) directs packets to the same host that generated them. This is useful
+for testing applications on a single host and for debugging.
+.PP
+The local CID obtained with
+.BR IOCTL_VM_SOCKETS_GET_LOCAL_CID
+can be used for the same purpose, but it is preferable to use
+.B VMADDR_CID_LOCAL .
 .SH ERRORS
 .TP
 .B EACCES
@@ -222,6 +231,11 @@ are valid.
 Support for VMware (VMCI) has been available since Linux 3.9.
 KVM (virtio) is supported since Linux 4.8.
 Hyper-V is supported since Linux 4.14.
+.PP
+VMADDR_CID_LOCAL is supported since Linux 5.6.
+Local communication in the guest and on the host is available since Linux 5.6.
+Previous versions only supported local communication within a guest
+(not on the host), and only with some transports (VMCI and virtio).
 .SH SEE ALSO
 .BR bind (2),
 .BR connect (2),