Re: [PATCH] evtchn: make EVTCHNOP_reset suitable for kexec

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 25/07/14 17:16, Andrew Cooper wrote:
> On 25/07/14 17:09, Jan Beulich wrote:
>>>>> On 25.07.14 at 17:48, <vkuznets@redhat.com> wrote:
>>> @@ -954,8 +955,20 @@ static long evtchn_reset(evtchn_reset_t *r)
>>>      if ( rc )
>>>          goto out;
>>>  
>>> -    for ( i = 0; port_is_valid(d, i); i++ )
>>> -        (void)__evtchn_close(d, i);
>>> +    for ( i = 1; port_is_valid(d, i); i++ )
>>> +    {
>>> +        /*
>>> +         * Leave all interdomain connections to Dom0 untouched as we need to
>>> +         * preserve store/console channels.
>>> +         */
>>> +        chn = evtchn_from_port(d, i);
>>> +        if ( chn->state != ECS_INTERDOMAIN ||
>>> +             chn->u.interdomain.remote_dom->domain_id != 0 )
>>> +            (void)__evtchn_close(d, i);
>>> +    }
>> You can't alter the behavior of an existing hypercall like this. Did
>> you at all check why it closes all channels, i.e. for what purpose
>> it got introduced?
>>
>> And apart from that blindly leaving all interdomain channels intact
>> doesn't seem reasonable either.
>>
>> Jan
>>
> I agree with your comment regarding interdomain channels.
>
> However, there absolutely nothing a domain can do to recover from a
> closed xenstore or console connection, as it is state set up by the
> toolstack at domain build time, rather than state set up during early boot.
>
> Whatever the intention of this hypercall originally, I can't see how a
> domain could possibly recover having used it.
>
> ~Andrew

Right - digging into the history shows that this was introduced 7 years
ago as a mixed toolstack or use-on-self hypercall.  In the toolstack
case, clearing all channels unconditionally is fine as the toolstack can
reset the xenstore/console event channels.

However, when using it with DOMID_SELF, the domain will clobber itself
irrevocably.

I think it would be acceptable to reset the console and store event
channels if and only if current->domain != d

~Andrew