linux-kernel - Re: [PATCH v2] virtio_console: Introduce an ID allocator for virtual console numbers

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Message-ID: <81612d82-7fc1-e2be-38a2-0db8314248cc@redhat.com>
Date:   Tue, 22 Nov 2022 12:12:42 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Cédric Le Goater <clg@...d.org>,
        Amit Shah <amit@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2] virtio_console: Introduce an ID allocator for virtual
 console numbers

On 14/11/2022 18.38, 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
> 
> Replace the next_vtermno global with an ID allocator and start the
> allocation at 1 as it is today. Also recycle IDs when a console port
> is removed.

Sounds like a good idea!

> @@ -1244,8 +1236,11 @@ 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;
> +	ret = ida_alloc_range(&vtermno_ida, 1, ~0, GFP_KERNEL);

Just cosmetics: I think you could use ida_alloc_min() instead.

> +	if (ret < 0)
> +		return ret;
>   
> +	port->cons.vtermno = ret;
>   	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);

What if this if (IS_ERR()) error happens? The code seems to return early in 
this case - shouldn't the ID be freed again in this case?

  Thomas