hvc/xen: Check console connection flag

When the console out buffer is filled, __write_console() will return 0
as it cannot send any data.  domU_write_console() will then spin in
`while (len)` as len doesn't decrement until xenconsoled attaches.  This
would block a domU and nullify the parallelism of Hyperlaunch until dom0
userspace starts xenconsoled, which empties the buffer.

Xen 4.21 added a connection field to the xen console page.  This is set
to XENCONSOLE_DISCONNECTED (1) when a domain is built, and xenconsoled
will set it to XENCONSOLE_CONNECTED (0) when it connects.

Update the hvc_xen driver to check the field.  When the field is
disconnected, drop the write with -ENOTCONN.  We only drop the write
when the field is XENCONSOLE_DISCONNECTED (1) to try for maximum
compatibility.  The Xen toolstack has historically zero initialized the
console, so it should see XENCONSOLE_CONNECTED (0) by default.  If an
implemenation used uninitialized memory, only checking for
XENCONSOLE_DISCONNECTED could have the lowest chance of not connecting.

This lets the hyperlaunched domU boot without stalling.  Once dom0
starts xenconsoled, xl console can be used to access the domU's hvc0.

Paritally sync console.h from xen.git to bring in the new field.

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Link: https://patch.msgid.link/20260318235326.14568-1-jason.andryuk@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: jandryuk

drivers/tty/hvc/hvc_xen.c

@@ -139,6 +139,9 @@ static ssize_t domU_write_console(uint32_t vtermno, const u8 *data, size_t len)
 	if (cons == NULL)
 		return -EINVAL;
 
+	if (cons->intf->connection == XENCONSOLE_DISCONNECTED)
+		return -ENOTCONN;
+
 	/*
 	 * Make sure the whole buffer is emitted, polling if
 	 * necessary.  We don't ever want to rely on the hvc daemon

include/xen/interface/io/console.h

@@ -19,6 +19,19 @@ struct xencons_interface {
     char out[2048];
     XENCONS_RING_IDX in_cons, in_prod;
     XENCONS_RING_IDX out_cons, out_prod;
+/*
+ * Flag values signaling from backend to frontend whether the console is
+ * connected.  i.e. Whether it will be serviced and emptied.
+ *
+ * The flag starts as disconnected.
+ */
+#define XENCONSOLE_DISCONNECTED 1
+/*
+ * The flag is set to connected when the backend connects and the console
+ * will be serviced.
+ */
+#define XENCONSOLE_CONNECTED    0
+    uint8_t connection;
 };
 
 #endif /* __XEN_PUBLIC_IO_CONSOLE_H__ */