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

[Cédric Le Goater]

On 11/14/22 09:57, Greg Kroah-Hartman wrote:
> 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@xxxxxxxx>
> > ---
> >   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?

yes that works.

> And why is this never decremented?

The driver would then need to track the id allocation ...

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

yes. we could also encode the tuple (vdev->index, port) using a bitmask,
possibly using 'max_nr_ports' to reduce the port width. VIRTCONS_MAX_PORTS
seems a bit big for this device and QEMU sets the #ports to 31.

An ida might be simpler. One drawback is that an id can be reused for a
different device/port tuple in case of an (unlikely) unplug/plug sequence.

Thanks,
C.

> thanks,
>
> greg k-h