Re: [PATCH v9 12/22] s390: vfio-ap: sysfs interfaces to configure control domains

[Halil Pasic <pasic@linux.ibm.com>]

On 08/21/2018 07:07 PM, Tony Krowiak wrote:
> On 08/21/2018 11:25 AM, Cornelia Huck wrote:
>> On Mon, 20 Aug 2018 13:41:32 -0400
>> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>>
>>> On 08/20/2018 10:23 AM, Cornelia Huck wrote:
>>>> On Mon, 13 Aug 2018 17:48:09 -0400
>>>> Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:
>>>>> From: Tony Krowiak <akrowiak@linux.ibm.com>
>>>>>
>>>>> Provides the sysfs interfaces for:
>>>>>
>>>>> 1. Assigning AP control domains to the mediated matrix device
>>>>>
>>>>> 2. Unassigning AP control domains from a mediated matrix device
>>>>>
>>>>> 3. Displaying the control domains assigned to a mediated matrix
>>>>>      device
>>>>>
>>>>> The IDs of the AP control domains assigned to the mediated matrix
>>>>> device are stored in an AP domain mask (ADM). The bits in the ADM,
>>>>> from most significant to least significant bit, correspond to
>>>>> AP domain numbers 0 to 255. On some systems, the maximum allowable
>>>>> domain number may be less than 255 - depending upon the host's
>>>>> AP configuration - and assignment may be rejected if the input
>>>>> domain ID exceeds the limit.
>>>> Please remind me of the relationship between control domains and usage
>>>> domains... IIRC, usage domains allow both requests and configuration,
>>>> while control domains allow only configuration, and are by convention a
>>>> superset of usage domains.
>>> A usage domain is a domain to which an AP command-request message can be
>>> submitted for processing. A control domain is a domain that can
>>> be changed by an AP command request message submitted to a usage domain.
>>> AP command request messages to configure a domain will contain the domain
>>> number of the domain to be modified. The AP firmware will check the
>>> control domain mask (ADM) and will allow the request to proceed only if
>>> the corresponding bit in the ADM is set.
>> Thanks to you and Halil for the explanation.
>>
>>>> Is there a hard requirement somewhere in there, or can the admin
>>>> cheerfully use different masks for usage domains and control domains
>>>> without the SIE choking on it?
>>> There is no hard requirement that control domains must be a superset of
>>> the usage domains, it is merely an architectural convention. AFAIK,
>>> SIE doesn't enforce this and will not break if the convention is not
>>> enforced externally. Having said that, you should note that the AQM
>>> and ADM masks configured for the mediated matrix device will be logically
>>> OR'd together to create the ADM stored in the CRYCB referenced from the
>>> guest's SIE state description. In other words, we are enforcing the
>>> convention in our software.
>> Hm, that's interesting, as Halil argued that we should not enforce it
>> in the kernel. Might be somewhat surprising as well. If that is really
>> the way to do it, this needs to be documented clearly.
> 
> This convention has been enforced by the kernel since v1. This is also
> enforced by both the LPAR as well as in z/VM. The following is from the
> PR/SM Planning Guide:
> 
> Control Domain
> A logical partition's control domains are those cryptographic domains for which remote secure
> administration functions can be established and administered from this logical partition. This
> logical partition’s control domains must include its usage domains. For each index selected in the
> usage domain index list, you must select the same index in the control domain index list
> 

IMHO this quote is quite a half-full half-empty cup one:
* it mandates the set of usage domains is a subset of the set
of the control domains, but
* it speaks of independent controls, namely about the 'usage domain index'
and the 'control domain index list' and makes the enforcement of the rule
a job of the administrator (instead of codifying it in the controls).

> 
> Consequently, I'm going to opt for ensuring this is clearly documented. Based on the fact you've
> requested clarification of many points described in this section of the doc, I
> think I'll try putting my meager skills as a wordsmith to work to hopefully clarify things.
> I'll run it by you when I complete that task to see if I've succeeded:)

I don't think just a doc update will do. Let me explain why.

What describe as "... note that the AQM and ADM masks configured for the
mediated matrix device will be logically OR'd together to create the ADM
stored in the CRYCB referenced from the guest's SIE state description."
is a gotcha at best. The member of struct ap_matrix and the member of the
respective apcb in the crycb are both called 'adm', but ap_matrix.adm is
not an ADM as we know it from the architecture, but rather ~ AQM & ADM.

I feel pretty strongly about this one. If we want to keep the enforcement
in the kernel, I guess, the assign_domain should set the bit corresponding
bit not only in ap_matrix.aqm but also in ap_matrix.adm. When the
ap_matrix is committed into the crycb no further manipulating the masks
should take place.

I don't feel strongly about whether to enforce this convention about AQM
and ADM in the kernel or not. Frankly, I don't know what is behind the
rule. Since I can't tell if any problems are to be expected if this
convention is violated, I would feel more comfortable if the rule was
accommodated higher in the management stack.

Regards,
Halil

> 
>>
>