Re: [PATCH v7 00/22] vfio-ap: guest dedicated crypto adapters

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On 07/27/2018 06:53 PM, Alex Williamson wrote:
> On Fri, 27 Jul 2018 12:59:50 +0200
> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> 
>> On 07/27/2018 10:38 AM, Cornelia Huck wrote:
>>> On Thu, 26 Jul 2018 21:54:07 +0200
>>> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
>>>    
>>>> * The mediated device gained an 'activate' attribute. Sharing conflicts are
>>>>   checked on activation now. If the device was not activated, the mdev
>>>>   open still implies activation. An active ap_matrix_mdev device claims
>>>>   it's resources -- an inactive does not.  
>>>
>>> This means we have a 'commit' workflow?  
>>
>> Yes. We want to be able to "overcommit" definitions. For example when you
>> have 2 guests that you never start at the same time. Then you can give both
>> guests the same disks. If you start at the same time, libvirt will complain.
>> Now: you want to do the same for matrixes. Allocation at definition time
>> would limit that flexibility. When we check at "commit" this allows overcommit.
> 
> I raised an eyebrow to this 'activate' attribute as well and I think we
> struggled through the same sort of thing when defining mdev initially
> with NVIDIA.  IIRC there was a proposal that mdev devices could
> effectively be overcommitted on the parent and only when they were
> opened, would the allocation count against the available instances.
> The trouble is then that libvirt has no guarantee that a given mdev
> device is usable.  I believe we decided that the creation of the mdev
> device is the point at which we want to reserve resources because it
> provides a better synchronization point.  I don't really see what
> advantage we have by having these matrices on 'standby', shouldn't
> userspace be able to manipulate these dynamically and on-demand of
> starting a VM?  Thanks,

We had this discussion as well and there is a case where not-predefining
things might complicate matters:
Daniel, please correct me if this is not so:
As far as I understand the libvirt folks want to have host devices and guest
instances decoupled. So a guest startup will not trigger a define of the mdev
instance. (instead it has to be a separate step). This might work with virsh
(but it now requires two steps as you can not predefine instances) but it
might break things like virt-manager.



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