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

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

Jan