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    SubjectRe: [PATCH v18 17/18] s390/Docs: new doc describing lock usage by the vfio_ap device driver
    On Mon, 14 Feb 2022 19:50:39 -0500
    Tony Krowiak <akrowiak@linux.ibm.com> wrote:

    > 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

    s/single instance of/singleton object of type/

    > +controls access to all fields contained within each matrix_mdev instance under
    > +the control of the vfio_ap device driver (matrix_dev->mdev_list).

    Are there matrix_mdev instances not under the control of the vfio_ap
    device driver?

    (MARK 1)

    > 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.

    This makes it look like for example struct vfio_ap_queue objects are out
    of scope.

    > +
    > +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

    Didn't say that access to fields of matrix_mdev instances is controlled
    by the matrix_dev->mdevs lock at (MARK 1)? How do the two statements
    mesh?

    > +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.
    > +

    It is very unclear what this lock is actually protecting, and when does
    it need to be taken.

    > +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.

    Just in a lockdep splat or in a deadlock?

    > The proper order for taking locks depends upon
    > +the operation taking place,

    That sounds very fishy! The whole point of having a locking order is
    preventing deadlocks if everybody sticks to the locking order. If there
    are exceptions, i.e. if we violate the locking order, we risk deadlocks.

    > 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.

    It looks a little odd that you describe all the operations here,
    especially in prosa.

    But hey the good thing is, none seem to violate the lock order.


    > +
    > +* 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.

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