[PATCH] vsock.7: add VMADDR_CID_LOCAL description

[Stefano Garzarella <sgarzare@redhat.com>]

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

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

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 man7/vsock.7 | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/man7/vsock.7 b/man7/vsock.7
index c5ffcf07d..d7dc37dcc 100644
--- a/man7/vsock.7
+++ b/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 +164,16 @@ 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
+The
+.B VMADDR_CID_LOCAL
+(1) can be used to address itself. In this case all packets are redirected
+to the same host that generated them. Useful for testing and 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 +232,8 @@ 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.
 .SH SEE ALSO
 .BR bind (2),
 .BR connect (2),
-- 
2.24.1