linux-kernel - Re: [PATCH v2] virtio_console: Introduce an ID allocator for virtual console numbers

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Date:   Tue, 22 Nov 2022 14:45:44 +0100
From:   Cédric Le Goater <clg@...d.org>
To:     Thomas Huth <thuth@...hat.com>, Amit Shah <amit@...nel.org>
CC:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <virtualization@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2] virtio_console: Introduce an ID allocator for virtual
 console numbers

On 11/22/22 12:12, Thomas Huth wrote:
> On 14/11/2022 18.38, 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
>>
>> Replace the next_vtermno global with an ID allocator and start the
>> allocation at 1 as it is today. Also recycle IDs when a console port
>> is removed.
> 
> Sounds like a good idea!
> 
>> @@ -1244,8 +1236,11 @@ 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;
>> +    ret = ida_alloc_range(&vtermno_ida, 1, ~0, GFP_KERNEL);
> 
> Just cosmetics: I think you could use ida_alloc_min() instead.
> 
>> +    if (ret < 0)
>> +        return ret;
>> +    port->cons.vtermno = ret;
>>       port->cons.hvc = hvc_alloc(port->cons.vtermno, 0, &hv_ops, PAGE_SIZE);
>>       if (IS_ERR(port->cons.hvc)) {
>>           ret = PTR_ERR(port->cons.hvc);
> 
> What if this if (IS_ERR()) error happens? The code seems to return early in this case - shouldn't the ID be freed again in this case?

Oh yes. I forgot that.

Thanks,

C.