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

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Date:   Mon, 14 Nov 2022 17:03:40 +0100
From:   Cédric Le Goater <clg@...d.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Amit Shah <amit@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        <virtualization@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio_console: Use an atomic to allocate virtual console
 numbers

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@...d.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 ...

> 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