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

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Message-ID: <ccf27ac3-c1de-aea6-689b-989781691de3@kaod.org>
Date:   Mon, 14 Nov 2022 18:00:13 +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 17:18, Greg Kroah-Hartman wrote:
> 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@...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 ...
> 
> 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?

A virtio console device can have multiple ports each being a /dev/hvcX
exposed in the guest OS. The "vportN" prefix identifies the virtio
device :

   # grep vtermno /sys/kernel/debug/virtio-ports/*
   /sys/kernel/debug/virtio-ports/vport1p0:console_vtermno: 2
   /sys/kernel/debug/virtio-ports/vport1p1:console_vtermno: 3
   /sys/kernel/debug/virtio-ports/vport1p2:console_vtermno: 4
   /sys/kernel/debug/virtio-ports/vport1p3:console_vtermno: 5
   /sys/kernel/debug/virtio-ports/vport2p0:console_vtermno: 1
   /sys/kernel/debug/virtio-ports/vport2p1:console_vtermno: 6
   /sys/kernel/debug/virtio-ports/vport2p2:console_vtermno: 7
   /sys/kernel/debug/virtio-ports/vport2p3:console_vtermno: 8

and "pX" the port within in the device. The naming is a bit confusing.

>> 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.

Let's go with the ida then.

Thanks,
C.