[PATCH] virtio_console: add console port to list before hvc_alloc()

[Foo Bar <muratseker@meta.com>]

From: Murat Seker <muratseker@meta.com>

init_port_console() calls hvc_alloc() before adding the port to
pdrvdata.consoles.  When the port is the preferred console (for example
console=hvc0), hvc_alloc() registers it, and register_console() replays
the log buffer through put_chars().  put_chars() looks the port up by
vtermno in pdrvdata.consoles, does not find it yet and returns -EPIPE,
so hvc discards every message printed before virtio-console probed.
The messages are still in the kernel log, but a host capturing the
virtio console never receives them.

Add the port to the list before calling hvc_alloc(), and remove it again
if hvc_alloc() fails.  The out_vq, outvq_lock and portdev used by
put_chars() are set up in add_port() before init_port_console() runs,
and the other users of cons.hvc check it through is_console_port().

Tested on x86_64 QEMU/KVM with KASAN, PROVE_LOCKING and DEBUG_LIST:
with console=hvc0 the virtio console now starts at [0.000000] instead of
at virtio-console probe; with hvc0 next to ttyS0 both carry the full
log; with earlycon (no replay) output is unchanged; and hot-adding and
removing extra virtconsole ports still works.  No new warnings.

Fixes: 38edf58d73c2 ("virtio: console: don't assume a single console port.")
Signed-off-by: Murat Seker <muratseker@meta.com>
---
 drivers/char/virtio_console.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 019bcae81af5..a31fdac65891 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1280,18 +1280,28 @@ static int init_port_console(struct port *port)
 		return ret;
 
 	port->cons.vtermno = ret;
+
+	/*
+	 * hvc_alloc() may register the console, which replays the log buffer
+	 * through put_chars().  put_chars() finds the port on this list, so
+	 * add it first or the replayed messages are dropped.
+	 */
+	spin_lock_irq(&pdrvdata_lock);
+	list_add_tail(&port->cons.list, &pdrvdata.consoles);
+	spin_unlock_irq(&pdrvdata_lock);
+
 	port->cons.hvc = hvc_alloc(port->cons.vtermno, 0, &hv_ops, PAGE_SIZE);
 	if (IS_ERR(port->cons.hvc)) {
 		ret = PTR_ERR(port->cons.hvc);
 		dev_err(port->dev,
 			"error %d allocating hvc for port\n", ret);
 		port->cons.hvc = NULL;
+		spin_lock_irq(&pdrvdata_lock);
+		list_del(&port->cons.list);
+		spin_unlock_irq(&pdrvdata_lock);
 		ida_free(&vtermno_ida, port->cons.vtermno);
 		return ret;
 	}
-	spin_lock_irq(&pdrvdata_lock);
-	list_add_tail(&port->cons.list, &pdrvdata.consoles);
-	spin_unlock_irq(&pdrvdata_lock);
 	port->guest_connected = true;
 
 	/* Notify host of port being opened */
-- 
2.53.0-Meta