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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

"Jan Beulich" <JBeulich@suse.com> writes:

>>>> 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?

Originally I though about introducing new hypercall to cleans up all
control blocks when FIFO-based event channel ABI is being used (see
"[PATCH RFC] evtchn: introduce EVTCHNOP_fifo_destroy hypercall"
email). Andrew and Ian convinced me to reuse EVTCHNOP_reset to be
ABI-agnostic here.

I'm not sure how to check the purpose of introduction but I did check
that EVTCHNOP_reset is not being used neither in Linux kernel nor in
http://xenbits.xensource.com/ext/win-pvdrivers/. The original commit
which introduces it is:
 commit 115209d91bcd3734ddaaf58a4a1cdbb4c44cd4fa
 Author: kfraser@localhost.localdomain <kfraser@localhost.localdomain>
 Date:   Fri Jan 19 17:20:57 2007 +0000

    [XEN] New event-channel reset operation.
    Plumbed through libxenctrl to python.
    
    From: Andrei Petrov <andrei.petrov@xensource.com>
    Signed-off-by: Keir Fraser <keir@xensource.com>

It would be great if you can point out EVTCHNOP_reset usages I'm not
aware of. We can revise the descision of reusing EVTCHNOP_reset in case
we break things.

>
> And apart from that blindly leaving all interdomain channels intact
> doesn't seem reasonable either.

I agree here, see my reply to Andrew. We need to preserve console/store
channels only and any suggestion on how to distinguish them from other
interdomain mappings to Dom0 on hypervisor side are welcome.

>
> Jan
>
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-- 
  Vitaly