xen/console: introduce conring_flush()

Introduce conring_flush() to ensure all messages kept in the internal
console ring are sent to all physical consoles (serial, VGA (x86))
after their initialization is completed.

Rename dump_console_ring_key to conring_dump_keyhandler to match the
notation for conring management symbols.

Resolves: https://gitlab.com/xen-project/xen/-/issues/184
Signed-off-by: Denis Mukhin <dmukhin@ford.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/drivers/char/console.c

@@ -452,23 +452,19 @@ void console_serial_puts(const char *s, size_t nr)
         serial_puts(sercon_handle, s, nr);
 }
 
-static void cf_check dump_console_ring_key(unsigned char key)
+/*
+ * Flush contents of the conring to the physical console devices.
+ */
+static int conring_flush(void)
 {
     uint32_t idx, len, sofar, c;
     unsigned int order;
     char *buf;
 
-    printk("'%c' pressed -> dumping console ring buffer (dmesg)\n", key);
-
-    /* create a buffer in which we'll copy the ring in the correct
-       order and NUL terminate */
     order = get_order_from_bytes(conring_size + 1);
     buf = alloc_xenheap_pages(order, 0);
     if ( buf == NULL )
-    {
-        printk("unable to allocate memory!\n");
-        return;
-    }
+        return -ENOMEM;
 
     c = conringc;
     sofar = 0;
@@ -486,6 +482,18 @@ static void cf_check dump_console_ring_key(unsigned char key)
     console_send(buf, sofar, CONSOLE_SERIAL | CONSOLE_VIDEO | CONSOLE_PV);
 
     free_xenheap_pages(buf, order);
+
+    return 0;
+}
+
+static void cf_check conring_dump_keyhandler(unsigned char key)
+{
+    int rc;
+
+    printk("'%c' pressed -> dumping console ring buffer (dmesg)\n", key);
+    rc = conring_flush();
+    if ( rc )
+        printk("failed to dump console ring buffer: %d\n", rc);
 }
 
 /*
@@ -1060,6 +1068,9 @@ void __init console_init_preirq(void)
     serial_set_rx_handler(sercon_handle, serial_rx);
     pv_console_set_rx_handler(serial_rx);
 
+    /* NB: send conring contents to all enabled physical consoles, if any */
+    conring_flush();
+
     /* HELLO WORLD --- start-of-day banner text. */
     nrspin_lock(&console_lock);
     __putstr(xen_banner());
@@ -1150,7 +1161,7 @@ void __init console_endboot(void)
     if ( opt_conswitch[1] == 'x' )
         console_rx = max_console_rx;
 
-    register_keyhandler('w', dump_console_ring_key,
+    register_keyhandler('w', conring_dump_keyhandler,
                         "synchronously dump console ring buffer (dmesg)", 0);
     register_irq_keyhandler('+', &do_inc_thresh,
                             "increase log level threshold", 0);