Re: [PATCH v17 00/15] s390/vfio-ap: dynamic configuration support

From: Tony Krowiak
Date: Mon Nov 22 2021 - 11:13:08 EST


On 10/21/21 11:23 AM, Tony Krowiak wrote:
The current design for AP pass-through does not support making dynamic
changes to the AP matrix of a running guest resulting in a few
deficiencies this patch series is intended to mitigate:

1. Adapters, domains and control domains can not be added to or removed
from a running guest. In order to modify a guest's AP configuration,
the guest must be terminated; only then can AP resources be assigned
to or unassigned from the guest's matrix mdev. The new AP
configuration becomes available to the guest when it is subsequently

2. The AP bus's /sys/bus/ap/apmask and /sys/bus/ap/aqmask interfaces can
be modified by a root user without any restrictions. A change to
either mask can result in AP queue devices being unbound from the
vfio_ap device driver and bound to a zcrypt device driver even if a
guest is using the queues, thus giving the host access to the guest's
private crypto data and vice versa.

3. The APQNs derived from the Cartesian product of the APIDs of the
adapters and APQIs of the domains assigned to a matrix mdev must
reference an AP queue device bound to the vfio_ap device driver. The
AP architecture allows assignment of AP resources that are not
available to the system, so this artificial restriction is not
compliant with the architecture.

4. The AP configuration profile can be dynamically changed for the linux
host after a KVM guest is started. For example, a new domain can be
dynamically added to the configuration profile via the SE or an HMC
connected to a DPM enabled lpar. Likewise, AP adapters can be
dynamically configured (online state) and deconfigured (standby state)
using the SE, an SCLP command or an HMC connected to a DPM enabled
lpar. This can result in inadvertent sharing of AP queues between the
guest and host.

5. A root user can manually unbind an AP queue device representing a
queue in use by a KVM guest via the vfio_ap device driver's sysfs
unbind attribute. In this case, the guest will be using a queue that
is not bound to the driver which violates the device model.

This patch series introduces the following changes to the current design
to alleviate the shortcomings described above as well as to implement
more of the AP architecture:

1. A root user will be prevented from making edits to the AP bus's
/sys/bus/ap/apmask or /sys/bus/ap/aqmask if the change would transfer
ownership of an APQN from the vfio_ap device driver to a zcrypt driver
while the APQN is assigned to a matrix mdev.

2. Allow a root user to hot plug/unplug AP adapters, domains and control
domains for a KVM guest using the matrix mdev via its sysfs
assign/unassign attributes.

4. Allow assignment of an AP adapter or domain to a matrix mdev even if
it results in assignment of an APQN that does not reference an AP
queue device bound to the vfio_ap device driver, as long as the APQN
is not reserved for use by the default zcrypt drivers (also known as
over-provisioning of AP resources). Allowing over-provisioning of AP
resources better models the architecture which does not preclude
assigning AP resources that are not yet available in the system. Such
APQNs, however, will not be assigned to the guest using the matrix
mdev; only APQNs referencing AP queue devices bound to the vfio_ap
device driver will actually get assigned to the guest.

5. Handle dynamic changes to the AP device model.

1. Rationale for changes to AP bus's apmask/aqmask interfaces:
Due to the extremely sensitive nature of cryptographic data, it is
imperative that great care be taken to ensure that such data is secured.
Allowing a root user, either inadvertently or maliciously, to configure
these masks such that a queue is shared between the host and a guest is
not only avoidable, it is advisable. It was suggested that this scenario
is better handled in user space with management software, but that does
not preclude a malicious administrator from using the sysfs interfaces
to gain access to a guest's crypto data. It was also suggested that this
scenario could be avoided by taking access to the adapter away from the
guest and zeroing out the queues prior to the vfio_ap driver releasing the
device; however, stealing an adapter in use from a guest as a by-product
of an operation is bad and will likely cause problems for the guest
unnecessarily. It was decided that the most effective solution with the
least number of negative side effects is to prevent the situation at the

2. Rationale for hot plug/unplug using matrix mdev sysfs interfaces:
Allowing a user to hot plug/unplug AP resources using the matrix mdev
sysfs interfaces circumvents the need to terminate the guest in order to
modify its AP configuration. Allowing dynamic configuration makes
reconfiguring a guest's AP matrix much less disruptive.

3. Rationale for allowing over-provisioning of AP resources:
Allowing assignment of AP resources to a matrix mdev and ultimately to a
guest better models the AP architecture. The architecture does not
preclude assignment of unavailable AP resources. If a queue subsequently
becomes available while a guest using the matrix mdev to which its APQN
is assigned, the guest will be given access to it. If an APQN
is dynamically unassigned from the underlying host system, it will
automatically become unavailable to the guest.

Change log v16-v17:
* Introduced a new patch (patch 1) to remove the setting of the pqap hook
in the group notifier callback. It is now set when the vfio_ap device
driver is loaded.

* Patch 6:
- Split the filtering of the APQNs and the control domains into
two functions and consolidated the vfio_ap_mdev_refresh_apcb and
vfio_ap_mdev_filter_apcb into one function named
vfio_ap_mdev_filter_matrix because the matrix is actually what is
being filtered.

- Removed ACK by Halil Pasic because of changes above; needs re-review.

* Introduced a new patch (patch 8) to keep track of active guests.

* Patch 9 (patch 8 in v16):
- Refactored locking to ensure KVM lock is taken before
matrix_dev->lock when hot plugging adapters, domains and
control domains.

- Removed ACK by Halil because of changes above; needs re-review.

* Patch 14 (patch 13 in v16):
- This patch has been redesigned to ensure proper locking order (i.e.,
taking kvm->lock before matrix_dev->lock).

- Removed Halil's Removed-by because of changes above; needs re-review.

Tony Krowiak (15):
s390/vfio-ap: Set pqap hook when vfio_ap module is loaded
s390/vfio-ap: use new AP bus interface to search for queue devices
s390/vfio-ap: move probe and remove callbacks to vfio_ap_ops.c
s390/vfio-ap: manage link between queue struct and matrix mdev
s390/vfio-ap: introduce shadow APCB
s390/vfio-ap: refresh guest's APCB by filtering APQNs assigned to mdev
s390/vfio-ap: allow assignment of unavailable AP queues to mdev device
s390/vfio-ap: keep track of active guests
s390/vfio-ap: allow hot plug/unplug of AP resources using mdev device
s390/vfio-ap: reset queues after adapter/domain unassignment
s390/ap: driver callback to indicate resource in use
s390/vfio-ap: implement in-use callback for vfio_ap driver
s390/vfio-ap: sysfs attribute to display the guest's matrix
s390/ap: notify drivers on config changed and scan complete callbacks
s390/vfio-ap: update docs to include dynamic config support

Documentation/s390/vfio-ap.rst | 492 ++++++---
arch/s390/include/asm/kvm_host.h | 10 +-
arch/s390/kvm/kvm-s390.c | 1 -
arch/s390/kvm/priv.c | 45 +-
drivers/s390/crypto/ap_bus.c | 241 ++++-
drivers/s390/crypto/ap_bus.h | 16 +
drivers/s390/crypto/vfio_ap_drv.c | 52 +-
drivers/s390/crypto/vfio_ap_ops.c | 1379 ++++++++++++++++++-------
drivers/s390/crypto/vfio_ap_private.h | 66 +-
9 files changed, 1714 insertions(+), 588 deletions(-)