Re: [PATCH] virtio_console: Use an atomic to allocate virtual console numbers

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Nov 14, 2022 at 09:07:52AM +0100, Cédric Le Goater wrote:
> When a virtio console port is initialized, it is registered as an hvc
> console using a virtual console number. If a KVM guest is started with
> multiple virtio console devices, the same vtermno (or virtual console
> number) can be used to allocate different hvc consoles, which leads to
> various communication problems later on.
> 
> This is also reported in debugfs :
> 
>   # grep vtermno /sys/kernel/debug/virtio-ports/*
>   /sys/kernel/debug/virtio-ports/vport1p1:console_vtermno: 1
>   /sys/kernel/debug/virtio-ports/vport2p1:console_vtermno: 1
>   /sys/kernel/debug/virtio-ports/vport3p1:console_vtermno: 2
>   /sys/kernel/debug/virtio-ports/vport4p1:console_vtermno: 3
> 
> Fix the issue with an atomic variable and start the first console
> number at 1 as it is today.
> 
> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> ---
>  drivers/char/virtio_console.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index 9fa3c76a267f..253574f41e57 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -58,12 +58,13 @@ struct ports_driver_data {
>  	 * We also just assume the first console being initialised was
>  	 * the first one that got used as the initial console.
>  	 */
> -	unsigned int next_vtermno;
> +	atomic_t next_vtermno;
>  
>  	/* All the console devices handled by this driver */
>  	struct list_head consoles;
>  };
> -static struct ports_driver_data pdrvdata = { .next_vtermno = 1};
> +
> +static struct ports_driver_data pdrvdata = { .next_vtermno = ATOMIC_INIT(0) };
>  
>  static DEFINE_SPINLOCK(pdrvdata_lock);
>  static DECLARE_COMPLETION(early_console_added);
> @@ -1244,7 +1245,7 @@ static int init_port_console(struct port *port)
>  	 * pointers.  The final argument is the output buffer size: we
>  	 * can do any size, so we put PAGE_SIZE here.
>  	 */
> -	port->cons.vtermno = pdrvdata.next_vtermno;
> +	port->cons.vtermno = atomic_inc_return(&pdrvdata.next_vtermno);

Why not use a normal ida/idr structure here?

And why is this never decremented?

and finally, why not use the value that created the "vportN" number
instead?

thanks,

greg k-h