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

Halil Pasic
2022-03-31T00:29:14Z

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.