Re: [PATCH v2] xen/console: Skip switching serial input to non existing domains

[Jan Beulich <jbeulich@suse.com>]

On 17.03.2023 10:32, Michal Orzel wrote:
> 
> 
> On 17/03/2023 09:36, Jan Beulich wrote:
>>
>>
>> On 16.03.2023 23:59, Stefano Stabellini wrote:
>>> On Thu, 16 Mar 2023, Jan Beulich wrote:
>>>> On 16.03.2023 11:26, Michal Orzel wrote:
>>>>> --- a/xen/drivers/char/console.c
>>>>> +++ b/xen/drivers/char/console.c
>>>>> @@ -490,7 +490,24 @@ static void switch_serial_input(void)
>>>>>      }
>>>>>      else
>>>>>      {
>>>>> -        console_rx++;
>>>>> +        unsigned int next_rx = console_rx + 1;
>>>>> +
>>>>> +        /* Skip switching serial input to non existing domains */
>>>>> +        while ( next_rx < max_init_domid + 1 )
>>>>> +        {
>>>>> +            struct domain *d = rcu_lock_domain_by_id(next_rx - 1);
>>>>> +
>>>>> +            if ( d )
>>>>> +            {
>>>>> +                rcu_unlock_domain(d);
>>>>> +                break;
>>>>> +            }
>>>>> +
>>>>> +            next_rx++;
>>>>> +        }
>>>>> +
>>>>> +        console_rx = next_rx;
>>>>> +
>>>>>          printk("*** Serial input to DOM%d", console_rx - 1);
>>>>>      }
>>>>
>>>> While at the first glance (when you sent it in reply to v1) it looked okay,
>>>> I'm afraid it really isn't: Please consider what happens when the last of
>>>> the DomU-s doesn't exist anymore. (You don't really check whether it still
>>>> exists, because the range check comes ahead of the existence one.) In that
>>>> case you want to move from second-to-last to Xen. I expect the entire
>>>> if/else construct wants to be inside the loop.
>>>
>>> I don't think we need another loop, just a check if we found a domain or
>>
>> I didn't say "another loop", but I suggested that the loop needs to be
>> around the if/else. Of course this can be transformed into equivalent
>> forms, like ...
>>
>>> not. E.g.:
>>>
>>>
>>>     unsigned int next_rx = console_rx + 1;
>>>
>>>     /* Skip switching serial input to non existing domains */
>>>     while ( next_rx < max_init_domid + 1 )
>>>     {
>>>         struct domain *d = rcu_lock_domain_by_id(next_rx - 1);
>>>
>>>         if ( d )
>>>         {
>>>             rcu_unlock_domain(d);
>>>             console_rx = next_rx;
>>>             printk("*** Serial input to DOM%d", console_rx - 1);
>>>             break;
>>>         }
>>>
>>>         next_rx++;
>>>     }
>>>
>>>     /* no domain found */
>>>     console_rx = 0;
>>>     printk("*** Serial input to Xen");
>>
>> ... what you suggest (or at least almost, because the way it's written
>> we'd always switch to Xen).
> 
> I would prefer a loop with if/else inside. If you are ok with the following code
> that handles all the cases, I will push a patch in a minute:

Looks roughly okay, but recall I said so also when you "pre-posted" the
previous version.

Jan