Re: [PATCH v7 03/15] s390/zcrypt: driver callback to indicate resource in use

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On 2020-04-07 21:20, Tony Krowiak wrote:
Introduces a new driver callback to prevent a root user from unbinding
an AP queue from its device driver if the queue is in use. The intent of
this callback is to provide a driver with the means to prevent a root user
from inadvertently taking a queue away from a guest and giving it to the
host while the guest is still using it.

How can we know, at this point if the guest uses or not the queue?
Do you want to say that this prevents to take away a queue when it is currently assigned to a VFIO device?
and with a guest currently using this VFIO device?

The callback will
be invoked whenever a change to the AP bus's sysfs apmask or aqmask
attributes would result in one or more AP queues being removed from its
driver. If the callback responds in the affirmative for any driver
queried, the change to the apmask or aqmask will be rejected with a device
in use error.

AFAIU you mean that Linux's driver's binding and unbinding mechanism is not sufficient to avoid this issue because unbind can not be refused by the driver.

The reason why we do not want a single queue to be removed from the VFIO driver is because the VFIO drivers works on a matrix, not on queues, and for the matrix to be consistent it needs to acquire all queues defined by the cross product of all APID and AQID assigned to the matrix.

This functionality is valid for the host as for the guests and is handled automatically by the firmware with the CRYCB. The AP bus uses QCI to retrieve the host CRYCB and build the hosts AP queues.

If instead to mix VFIO CRYCB matrix handling and queues at the same level inside the AP bus we separate these different firmware entities in two different software entities.

If we make the AP bus sit above a CRYCB/Matrix bus, and in the way virtualize the QCI and test AP queue instructions: - we can directly pass a matrix device to the guest though a VFIO matrix device
- the consistence will be automatic
- the VFIO device and parent device will be of the same kind which would make the design much more clearer. - there will be no need for these callback because the consistence of the matrix will be guaranteed by firmware



For this patch, only non-default drivers will be queried. Currently,
there is only one non-default driver, the vfio_ap device driver.

You mean that the admin may take queues away from the "default driver", while the queue is in use, to give it to an other driver?
Why is it to avoid in one way and not in the other way?

The
vfio_ap device driver manages AP queues passed through to one or more
guests

I read this as if a queue may be passed to several guest...
please, rephrase or explain.

and we don't want to unexpectedly take AP resources away from
guests which are most likely independently administered.

When you say "independently administered", you mean as a second admin inside the host, don't you?

Regards,
Pierre

--
Pierre Morel
IBM Lab Boeblingen



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