Re: [PATCHv1] evtchn: don't reuse ports that are still "busy"

[David Vrabel <david.vrabel@citrix.com>]

On 01/12/15 12:49, Jan Beulich wrote:
>>>> On 30.11.15 at 18:59, <david.vrabel@citrix.com> wrote:
>> --- a/xen/common/event_channel.c
>> +++ b/xen/common/event_channel.c
>> @@ -170,7 +170,8 @@ static int get_free_port(struct domain *d)
>>      {
>>          if ( port > d->max_evtchn_port )
>>              return -ENOSPC;
>> -        if ( evtchn_from_port(d, port)->state == ECS_FREE )
>> +        chn = evtchn_from_port(d, port);
>> +        if ( chn->state == ECS_FREE && !evtchn_port_is_busy(d, chn) )
> 
> Despite the reasonable arguments you give this looks very wrong:
> How can a free port still be busy? Could we have a new ECS_* and
> require guests to notify the hypervisor when they unlinked an
> already closed port (while "close" would transition busy ports into
> that new state)?

I would look at it as: The channel object is free, but the corresponding
ABI specific port object is busy.  So it doesn't seem unreasonable to
check the state of both objects.

What you suggest (adding an additional call) would break all existing
guests that would not make the unlinked call, leaving the event channel
in a state where it cannot be reused.

David