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@xxxxxxxxxxxxx> --- 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