[PATCH v18 17/18] s390/Docs: new doc describing lock usage by the vfio_ap device driver

Tony Krowiak
2022-02-15T00:51:24Z

Introduces a new document describing the locks used by the vfio_ap device
driver and how to use them so as to avoid lockdep reports and deadlock
situations.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
---
Documentation/s390/vfio-ap-locking.rst | 389 +++++++++++++++++++++++++
1 file changed, 389 insertions(+)
create mode 100644 Documentation/s390/vfio-ap-locking.rst

diff --git a/Documentation/s390/vfio-ap-locking.rst b/Documentation/s390/vfio-ap-locking.rst
new file mode 100644
index 000000000000..10abbb6d6089
--- /dev/null
+++ b/Documentation/s390/vfio-ap-locking.rst
@@ -0,0 +1,389 @@
+======================
+VFIO AP Locks Overview
+======================
+This document describes the locks that are pertinent to the secure operation
+of the vfio_ap device driver. Throughout this document, the following variables
+will be used to denote instances of the structures herein described:
+
+struct ap_matrix_dev *matrix_dev;
+struct ap_matrix_mdev *matrix_mdev;
+struct kvm *kvm;
+
+The Matrix Devices Lock (drivers/s390/crypto/vfio_ap_private.h)
+--------------------------------------------------------------
+
+struct ap_matrix_dev {
+ ...
+ struct list_head mdev_list;
+ struct mutex mdevs_lock;
+ ...
+}
+
+The Matrix Devices Lock (matrix_dev->mdevs_lock) is implemented as a global
+mutex contained within the single instance of struct ap_matrix_dev. This lock
+controls access to all fields contained within each matrix_mdev instance under
+the control of the vfio_ap device driver (matrix_dev->mdev_list). This lock must
+be held while reading from, writing to or using the data from a field contained
+within a matrix_mdev instance representing one of the vfio_ap device driver's
+mediated devices.
+
+The KVM Lock (include/linux/kvm_host.h)
+---------------------------------------
+
+struct kvm {
+ ...
+ struct mutex lock;
+ ...
+}
+
+The KVM Lock (kvm->lock) controls access to the state data for a KVM guest. This
+lock must be held by the vfio_ap device driver while one or more AP adapters,
+domains or control domains are being plugged into or unplugged from the guest.
+
+The vfio_ap device driver registers a function to be notified when the pointer
+to the kvm instance has been set. The KVM pointer is passed to the handler by
+the notifier and is stored in the in the matrix_mdev instance
+(matrix_mdev->kvm = kvm) containing the state of the mediated device that has
+been passed through to the KVM guest.
+
+The Guests Lock (drivers/s390/crypto/vfio_ap_private.h)
+-----------------------------------------------------------
+
+struct ap_matrix_dev {
+ ...
+ struct list_head mdev_list;
+ struct mutex guests_lock;
+ ...
+}
+
+The Guests Lock (matrix_dev->guests_lock) controls access to the
+matrix_mdev instances (matrix_dev->mdev_list) that represent mediated devices
+that hold the state for the mediated devices that have been passed through to a
+KVM guest. This lock must be held:
+
+1. To control access to the KVM pointer (matrix_mdev->kvm) while the vfio_ap
+ device driver is using it to plug/unplug AP devices passed through to the KVM
+ guest.
+
+2. To add matrix_mdev instances to or remove them from matrix_dev->mdev_list.
+ This is necessary to ensure the proper locking order when the list is perused
+ to find an ap_matrix_mdev instance for the purpose of plugging/unplugging
+ AP devices passed through to a KVM guest.
+
+ For example, when a queue device is removed from the vfio_ap device driver,
+ if the adapter is passed through to a KVM guest, it will have to be
+ unplugged. In order to figure out whether the adapter is passed through,
+ the matrix_mdev object to which the queue is assigned will have to be
+ found. The KVM pointer (matrix_mdev->kvm) can then be used to determine if
+ the mediated device is passed through (matrix_mdev->kvm != NULL) and if so,
+ to unplug the adapter.
+
+It is not necessary to take the Guests Lock to access the KVM pointer if the
+pointer is not used to plug/unplug devices passed through to the KVM guest;
+however, in this case, the Matrix Devices Lock (matrix_dev->mdevs_lock) must be
+held in order to access the KVM pointer since it set and cleared under the
+protection of the Matrix Devices Lock. A case in point is the function that
+handles interception of the PQAP(AQIC) instruction sub-function. This handler
+needs to access the KVM pointer only for the purposes of setting or clearing IRQ
+resources, so only the matrix_dev->mdevs_lock needs to be held.
+
+The PQAP Hook Lock (arch/s390/include/asm/kvm_host.h)
+-----------------------------------------------------
+
+typedef int (*crypto_hook)(struct kvm_vcpu *vcpu);
+
+struct kvm_s390_crypto {
+ ...
+ struct rw_semaphore pqap_hook_rwsem;
+ crypto_hook *pqap_hook;
+ ...
+};
+
+The PQAP Hook Lock is a r/w semaphore that controls access to the function
+pointer of the handler (*kvm->arch.crypto.pqap_hook) to invoke when the
+PQAP(AQIC) instruction sub-function is intercepted by the host. The lock must be
+held in write mode when pqap_hook value is set, and in read mode when the
+pqap_hook function is called.
+
+Locking Order
+-------------
+
+If the various locks are not taken in the proper order, it could potentially
+result in a lockdep splat. The proper order for taking locks depends upon
+the operation taking place, but in general the Guests Lock
+(matrix_dev->guests_lock) must be taken outside of the KVM Lock (kvm->lock)
+which in turn must be taken outside of the Matrix Devices Lock
+(matrix_dev->mdevs_lock).
+
+The following describes the various operations under which the various locks are
+taken, the purpose for taking them and the order in which they must be taken.
+
+* Operations: Setting or clearing the KVM pointer (matrix_mdev->kvm):
+
+ 1. PQAP Hook Lock (kvm->arch.crypto.pqap_hook_rwsem):
+
+ This semaphore must be held in write mode while setting or clearing the
+ reference to the function pointer (kvm->arch.crypt.pqap_hook) to call
+ when the PQAP(AQIC) instruction sub-function is intercepted by the host.
+ The function pointer is set when the KVM pointer is being set and
+ cleared when the KVM pointer is being cleared.
+
+ 2.Guests Lock (matrix_dev->guests_lock):
+
+ This mutex must be held while accessing the KVM pointer
+ (matrix_mdev->kvm) to plug/unplug AP devices passed through to the
+ KVM guest
+
+ 3. KVM Lock (kvm->lock):
+
+ This mutex must be held while the AP devices passed through to the KVM
+ guest are plugged/unplugged.
+
+ 4. Matrix Devices Lock (matrix_dev->mdevs_lock)
+
+ This lock must be held to prevent access to the matrix_mdev state
+ while writing/reading state values during the operation.
+
+* Operations: Assign or unassign an adapter, domain or control domain of a
+ mediated device under the control of the vfio_ap device driver:
+
+ 1. Guests Lock (matrix_dev->guests_lock):
+
+ This mutex must be held while accessing the KVM pointer
+ (matrix_dev->kvm) to plug/unplug AP devices passed through to the
+ KVM guest as a result of the assignment/unassignment operation.
+ Assignment of an AP device may result in additional queue devices
+ or control domains being plugged into the guest. Similarly, unassignment
+ may result in unplugging queue devices or control domains from the
+ guest
+
+ 3. KVM Lock (matrix_mdev->kvm->lock):
+
+ This mutex must be held while the AP devices passed through to the KVM
+ guest are plugged in or unplugged.
+
+ 4. Matrix Devices Lock (matrix_dev->mdevs_lock)
+
+ This lock must be held to prevent access to the matrix_mdev state
+ while writing/reading state values during the operation. For example, to
+ determine which AP devices need to be plugged/unplugged, the lock
+ must be held to prevent other operations from changing the data used
+ to construct the guest's AP configuration.
+
+* Operations: Probe or remove an AP queue device:
+
+ When a queue device is bound to the vfio_ap device driver, the driver's probe
+ callback is invoked. Similarly, when a queue device is unbound from the
+ driver it's remove callback is invoked. The probe and remove functions will
+ take locks in the following order:
+
+ 1. Guests Lock (matrix_dev->guests_lock):
+
+ This mutex must be held for the duration of this operation.
+
+ At the time of the operation, the vfio_ap device driver will only have
+ the APQN of the queue being probed or removed, so the
+ matrix_dev->mdevs_list must be perused to locate the matrix_mdev
+ instance to which the queue is assigned. The Guests Lock must be held
+ during this time to prevent the list from being changed while processing
+ the probe/remove.
+
+ Once the matrix_mdev is found, the operation must determine whether the
+ mediated device is passed through to a guest (matrix_mdev->kvm != NULL),
+ then use the KVM pointer to perform the plug/unplug operation. Here
+ again, the lock must be held to prevent other operations from accessing
+ the KVM pointer for the same purpose.
+
+ 2. KVM Lock (kvm->lock):
+
+ This mutex must be held while the AP devices passed through to the KVM
+ guest are plugged in or unplugged to prevent other operations from
+ accessing the guest's state while it is in flux.
+
+ 3. Matrix Devices Lock (matrix_dev->mdevs_lock)
+
+ This lock must be held to prevent access to the matrix_mdev state
+ while writing/reading state values during the operation, such as the
+ masks used to construct the KVM guest's AP configuration.
+
+* Operations: Probe or remove a mediated device:
+
+ 1. Guests Lock (matrix_dev->guests_lock):
+
+ This mutex must be held while adding the matrix_mdev to the
+ matrix_dev->mdev_list during the probe operation or when removing it
+ from the list during the remove operation. This is to prevent access by
+ other functions that must traverse the list to find a matrix_mdev for
+ the purpose of plugging/unplugging AP devices passed through to a KVM
+ guest (i.e., probe/remove queue callbacks), while the list is being
+ modified.
+
+ 2. Matrix Devices Lock (matrix_dev->mdevs_lock)
+
+ This lock must be held to prevent access to the matrix_mdev state
+ while writing/reading state values during the probe or remove operations
+ such as initializing the hashtable of queue devices
+ (matrix_mdev->qtable.queues) assigned to the matrix_mdev.
+
+* Operation: Handle interception of the PQAP(AQIC) instruction sub-function:
+
+ 1. PQAP Hook Lock (kvm->arch.crypto.pqap_hook_rwsem)
+
+ This semaphore must be held in read mode while retrieving the function
+ pointer (kvm->arch.crypto.pqap_hook) and executing the function that
+ handles the interception of the PQAP(AQIC) instruction sub-function by
+ the host.
+
+ 2. Matrix Devices Lock (matrix_dev->mdevs_lock)
+
+ This lock must be held to prevent access to the matrix_mdev state
+ while writing/reading state values during the execution of the
+ PQAP(AQIC) instruction sub-function interception handler. For example,
+ the handler must iterate over the matrix_mdev->qtable.queues hashtable
+ to find the vfio_ap_queue object representing the queue for which
+ interrupts are being enabled or disabled.
+
+ Note: It is not necessary to take the Guests Lock (matrix_dev->guests_lock)
+ or the KVM Lock (matrix_mdev->kvm->lock) because the KVM pointer
+ will not be accessed to plug/unplug AP devices passed through to the
+ guest; it will only be used to allocate or free resources for processing
+ interrupts.
+
+* Operation: Handle AP configuration changed notification:
+
+ The vfio_ap device driver registers a callback function to be notified when
+ the AP bus detects that the host's AP configuration has changed. This can
+ occur due to the addition or removal of AP adapters, domains or control
+ domains via an SE or HMC connected to a DPM enabled LPAR. The objective of the
+ handler is to remove the queues no longer accessible via the host in bulk
+ rather than one queue at a time via the driver's queue device remove callback.
+ The locks and the order in which they must be taken by this operation are:
+
+ 1. Guests Lock (matrix_dev->guests_lock):
+
+ This mutex must be held for the duration of the operation to:
+
+ * Iterate over the matrix_dev->mdev_list to find each matrix_mdev from
+ which a queue device to be removed is assigned and prevent other
+ operations from modifying the list while processing the affected
+ matrix_mdev instances.
+
+ * Prevent other operations from acquiring access to the KVM pointer in
+ each affected matrix_mdev instance (matrix_mdev->kvm) for the purpose
+ of plugging/unplugging AP devices passed through to the KVM guest via
+ that instance.
+
+2. KVM Lock (kvm->lock):
+
+ This mutex must be held for each affected matrix_mdev instance while
+ the AP devices passed through to the KVM guest are unplugged to prevent
+ other operations from accessing the guest's state while it is in flux.
+
+ Note: This lock must be re-acquired for each matrix_mdev instance.
+
+ 3. Matrix Devices Lock (matrix_dev->mdevs_lock)
+
+ This lock must be held for each affected matrix_mdev to prevent access
+ to the matrix_mdev state while writing/reading state values during the
+ operation, such as the masks used to construct the KVM guest's AP
+ configuration.
+
+ Note: This lock must be re-acquired for each matrix_mdev instance.
+
+Operation: Handle AP bus scan complete notification:
+
+ The vfio_ap device driver registers a callback function to be notified when
+ the AP bus scan completes after detecting the addition or removal of AP
+ adapters, domains or control domains. The objective of the handler is t
+ add the new queues accessible via the host in bulk rather than one queue
+ at a time via the driver's queue device probe callback. The locks and the
+ order in which they must be taken by this operation are:
+
+ 1. Guests Lock (matrix_dev->guests_lock):
+
+ This mutex must be held for the duration of the operation to:
+
+ * Iterate over the matrix_dev->mdev_list to find each matrix_mdev to
+ which a queue device added is assigned and prevent other operations
+ from modifying the list while processing each affected matrix_mdev
+ object.
+
+ * Prevent other operations from acquiring access to the KVM pointer in
+ each affected matrix_mdev instance (matrix_mdev->kvm) for the purpose
+ of plugging/unplugging AP devices passed through to the KVM guest via
+ that instance.
+
+ 2. KVM Lock (kvm->lock):
+
+ This mutex must be held for each affected matrix_mdev instance while
+ the AP devices passed through to the KVM guest are plugged in to prevent
+ other operations from accessing the guest's state while it is in flux.
+
+ Note: This lock must be re-acquired for each matrix_mdev instance.
+
+ 3. Matrix Devices Lock (matrix_dev->mdevs_lock):
+
+ This lock must be held for each affected matrix_mdev to prevent access
+ to the matrix_mdev state while writing/reading state values during the
+ operation, such as the masks used to construct the KVM guest's AP
+ configuration.
+
+ Note: This lock must be re-acquired for each matrix_mdev instance.
+
+Operation: Handle resource in use query:
+
+ The vfio_ap device driver registers a callback function with the AP bus to be
+ called when changes to the bus's sysfs /sys/bus/ap/apmask or
+ /sys/bus/ap/aqmask attributes would result in one or more AP queue devices
+ getting unbound from the vfio_ap device driver to verify none of them are in
+ use by the driver (i.e., assigned to a matrix_mdev instance). If this function
+ is called while an adapter or domain is also being assigned to a mediated
+ device, this could result in a deadlock; for example:
+
+ 1. A system administrator assigns an adapter to a mediated device under the
+ control of the vfio_ap device driver. The driver will need to first take
+ the matrix_dev->guests_lock to potentially hot plug the adapter into
+ the KVM guest.
+ 2. At the same time, a system administrator sets a bit in the sysfs
+ /sys/bus/ap/ap_mask attribute. To complete the operation, the AP bus
+ must:
+ a. Take the ap_perms_mutex lock to update the object storing the values
+ for the /sys/bus/ap/ap_mask attribute.
+ b. Call the vfio_ap device driver's in-use callback to verify that no
+ queues now being reserved for the default zcrypt drivers are
+ in use by the vfio_ap device driver. To do the verification, the in-use
+ callback function takes the matrix_dev->guests_lock, but has to wait
+ because it is already held by the operation in 1 above.
+ 3. The vfio_ap device driver calls an AP bus function to verify that the
+ new queues resulting from the assignment of the adapter in step 1 are
+ not reserved for the default zcrypt device driver. This AP bus function
+ tries to take the ap_perms_mutex lock but gets stuck waiting for the
+ it due to step 2a above.
+
+ Consequently, we have the following deadlock situation:
+
+ matrix_dev->guests_lock locked (1)
+ ap_perms_mutex lock locked (2a)
+ Waiting for matrix_dev->lock (2b) which is currently held (1)
+ Waiting for ap_perms_mutex lock (3) which is currently held (2a)
+
+ To prevent the deadlock scenario, the in_use operation will take the
+ required locks using the mutex_trylock() function and if the lock can not be
+ acquired will terminate and return -EBUSY to indicate the driver is busy
+ processing another request.
+
+ The locks required to respond to the handle resource in use query and the
+ order in which they must be taken are:
+
+ 1. Guests Lock (matrix_dev->guests_lock):
+
+ This mutex must be held for the duration of the operation to iterate over the
+ matrix_dev->mdev_list to determine whether any of the queues to be unbound
+ are assigned to a matrix_mdev instance.
+
+ 2. Matrix Devices Lock (matrix_dev->mdevs_lock):
+
+ This mutex must be held for the duration of the operation to ensure that the
+ AP configuration of each matrix_mdev instance does not change while verifying
+ that none of the queue devices to be removed from the vfio_ap driver are
+ assigned to it.
--
2.31.1