Re: [PATCH] console: Do not duplicate early printk messages on conring flush

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 17 Jun 2025, Michal Orzel wrote:
> Commit f6d1bfa16052 introduced flushing conring in console_init_preirq().
> However, when CONFIG_EARLY_PRINTK is enabled, the early boot messages
> had already been sent to serial before main console initialization. This
> results in all the early boot messages being duplicated.
> 
> Change conring_flush() to accept argument listing devices to which to
> flush conring. We don't want to send to serial at console initialization
> when using early printk, but we want these messages to be send at conring
> dump triggered by keyhandler.
> 
> Fixes: f6d1bfa16052 ("xen/console: introduce conring_flush()")
> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>  xen/drivers/char/console.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
> index 9a9836ba91e7..5879e31786ba 100644
> --- a/xen/drivers/char/console.c
> +++ b/xen/drivers/char/console.c
> @@ -453,9 +453,9 @@ void console_serial_puts(const char *s, size_t nr)
>  }
>  
>  /*
> - * Flush contents of the conring to the physical console devices.
> + * Flush contents of the conring to the selected console devices.
>   */
> -static int conring_flush(void)
> +static int conring_flush(unsigned int flags)
>  {
>      uint32_t idx, len, sofar, c;
>      unsigned int order;
> @@ -479,7 +479,7 @@ static int conring_flush(void)
>          c += len;
>      }
>  
> -    console_send(buf, sofar, CONSOLE_SERIAL | CONSOLE_VIDEO | CONSOLE_PV);
> +    console_send(buf, sofar, flags);
>  
>      free_xenheap_pages(buf, order);
>  
> @@ -491,7 +491,7 @@ 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();
> +    rc = conring_flush(CONSOLE_SERIAL | CONSOLE_VIDEO | CONSOLE_PV);
>      if ( rc )
>          printk("failed to dump console ring buffer: %d\n", rc);
>  }
> @@ -1042,6 +1042,7 @@ void __init console_init_preirq(void)
>  {
>      char *p;
>      int sh;
> +    unsigned int flags = CONSOLE_SERIAL | CONSOLE_VIDEO | CONSOLE_PV;
>  
>      serial_init_preirq();
>  
> @@ -1084,8 +1085,15 @@ 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();
> +    /*
> +     * NB: send conring contents to all enabled physical consoles, if any.
> +     * Skip serial if CONFIG_EARLY_PRINTK is enabled, which means the early
> +     * messages have already been sent to serial.
> +     */
> +    if ( IS_ENABLED(CONFIG_EARLY_PRINTK) )
> +        flags &= ~CONSOLE_SERIAL;
> +
> +    conring_flush(flags);
>  
>      /* HELLO WORLD --- start-of-day banner text. */
>      nrspin_lock(&console_lock);
> -- 
> 2.25.1
>