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Subject: Re: [PATCH v9 12/22] s390: vfio-ap: sysfs interfaces to configure
 control domains
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     Halil Pasic <pasic@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
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From:   Tony Krowiak <akrowiak@linux.ibm.com>
Date:   Mon, 27 Aug 2018 09:47:58 -0400
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On 08/27/2018 04:33 AM, Cornelia Huck wrote:
> On Thu, 23 Aug 2018 10:16:59 -0400
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> On 08/23/2018 06:25 AM, Cornelia Huck wrote:
>>> On Wed, 22 Aug 2018 15:16:19 -0400
>>> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>>>   
>>>> One of the things I suggested in a private conversation with Christian
>>>> earlier
>>>> today was to provide an additional rw sysfs attribute - a boolean - that
>>>> indicates
>>>> whether all usage domains should also be control domains. The default
>>>> could be
>>>> true. This would allow one to configure guests with usage-only domains
>>>> as well
>>>> as satisfy the convention.
>>> Would this additional attribute then control "add usage domains to the
>>> list of control domains automatically", or "don't allow to add a usage
>>> domain if it has not already been added as a control domain"?
>> It was just a proposal that wasn't really discussed at all, but this
>> attribute would add usage domains to the list of control domains
>> automatically if set to one. That would be the default behavior which
>> would be turned off by manually setting it to zero.
> If we want to do something like that, having it add the usage domains
> automatically sounds like the more workable alternative. What I like
> about this is that we make it explicit that we change the masks beyond
> what the admin explicitly configured, and provide a knob to turn off
> that behaviour.

So, are you saying I should go ahead and implement this?

>
>>> One thing I'm still unsure about is how libvirt comes into the picture
>>> here. Will it consume the setting, or actively manipulate it?
>> libvirt is not in the picture. The only thing libvirt does is identify
>> which mediated matrix device (i.e., UUID) is to be used by the guest.
>>
>>> [In general, I'm not very clear about how libvirt will interact with the
>>> whole infrastructure...]
>> The libvirt domain XML will only specify the UUID of the matrix device
>> to be used by the guest. Consequently, libvirt will insert the device
>> statement for the vfio-ap device into the QEMU command line:
>>
>> -device vfio-ap,syfsdev={path-to-mdev}
>>
>> All configuration of the mediated device, including the proposed attribute
>> above, is handled via sysfs attributes.
> So, basically, libvirt will only consume what the admin has already
> configured?

Yes.

>