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Subject: Re: [PATCH v7 03/15] s390/zcrypt: driver callback to indicate
 resource in use
To:     Tony Krowiak <akrowiak@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
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        alex.williamson@redhat.com, kwankhede@nvidia.com,
        jjherne@linux.ibm.com, fiuczy@linux.ibm.com
References: <20200407192015.19887-1-akrowiak@linux.ibm.com>
 <20200407192015.19887-4-akrowiak@linux.ibm.com>
From:   Pierre Morel <pmorel@linux.ibm.com>
Message-ID: <75bcbc06-f38f-1aff-138f-5d2a2dd3f7b6@linux.ibm.com>
Date:   Mon, 27 Apr 2020 10:20:40 +0200
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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