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Message-ID: <b0503354-2d1e-a93d-a6a5-6f6a1f55f0e2@kaod.org>
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
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