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

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

On Mon, Nov 14, 2022 at 05:03:40PM +0100, Cédric Le Goater wrote:
> 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@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?
> 
> yes that works.
> 
> > And why is this never decremented?
> 
> The driver would then need to track the id allocation ...

That's what an ida/idr does.

> > 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,

No need for that, you already have a unique number in the name above,
why not use that?

> possibly using 'max_nr_ports' to reduce the port width.

Why is that an issue?  Maybe I am confused as to what this magic
"vtermno" is here.  Who uses it and why is the vportN number not
sufficient?

> 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.

What's wrong with that?  We do not have persistent device names from
within the kernel.

thanks,

greg k-h
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