linux-kernel - Re: [PATCH] zcrypt: handle AP Info notification from CHSC SEI command

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Date:   Fri, 1 Feb 2019 10:50:13 -0500
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        sebott@...ux.ibm.com, oberpar@...ux.ibm.com, freude@...ux.ibm.com,
        pmorel@...ux.ibm.com, pasic@...ux.ibm.com
Subject: Re: [PATCH] zcrypt: handle AP Info notification from CHSC SEI command

On 2/1/19 9:35 AM, Cornelia Huck wrote:
> On Thu, 31 Jan 2019 18:50:57 -0500
> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> 
>> On 1/31/19 4:55 AM, Cornelia Huck wrote:
>>> On Wed, 30 Jan 2019 12:48:46 -0500
>>> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> 
>>> Two questions:
>>> - Does the event cover _any_ change to the AP configuration, or can the
>>>     periodic scan detect changes that are not signaled?
>>
>> It can detect any change, such as a change to the CRYCB masks.
> 
> Nice. I suppose we can not rely on those messages being generated,
> though, and therefore need to keep the periodic scan...

I don't know how the CRYCB can be changed dynamically on the host, but
hot plug for a guest changes it dynamically. Down the road, we may
send a CHSC SEI AP Configuration event to let the guest know. I don't 
know if there may be other AP config changes that can occur without this
event being posted, so it is probably a good idea to keep the scan. It
certainly doesn't hurt anything to do so.

> 
>>
>>> - Do we want to generate such an event in QEMU on plugging/unplugging
>>>     the vfio-ap device?
>>
>> We've discussed this quite a bit internally and decided not to implement
>> that at this time. We will address it as a future enhancement.
> 
> Ok, but I think it would be nice to have.

Duly noted, but that discussion is outside of scope for this patch.

> 
>>>> diff --git a/drivers/s390/cio/chsc.c b/drivers/s390/cio/chsc.c
>>>> index a0baee25134c..dccccc337078 100644
>>>> --- a/drivers/s390/cio/chsc.c
>>>> +++ b/drivers/s390/cio/chsc.c
>>>> @@ -586,6 +586,15 @@ static void chsc_process_sei_scm_avail(struct chsc_sei_nt0_area *sei_area)
>>>>    			      " failed (rc=%d).\n", ret);
>>>>    }
>>>>    
>>>> +static void chsc_process_sei_ap_cfg_chg(struct chsc_sei_nt0_area *sei_area)
>>>> +{
>>>> +	CIO_CRW_EVENT(3, "chsc: ap config changed\n");
>>>> +	if (sei_area->rs != 5)
>>>> +		return;
>>>
>>> I'm guessing that a reporting source of 5 means ap, right? (The code is
>>> silent on all those magic rs values :/)
>>
>> The 5 indicates the accessibility of one or more adjunct processors has
>> changed. The reason this gets called is because the CC sent with the
>> instruction indicates the AP configuration has changed, so the reporting
>> belongs where it is. There is only one RS associated with it.
> 
> So if we'd ever get there anything but rs == 5, it would be a hardware
> or hypervisor bug? Then the code makes sense, I guess.

I have no idea if that is possible, but this follows the architecture.

> 
>>
>>>
>>> If so, should the debug logging be moved after the check?
>>
>> covered in the response above.
>>
>>>    
>>>> +
>>>> +	ap_bus_cfg_chg();
>>>> +}
>>>> +
>