Date:   Mon, 4 Apr 2022 17:34:48 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, jjherne@...ux.ibm.com, freude@...ux.ibm.com,
        borntraeger@...ibm.com, cohuck@...hat.com, mjrosato@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com
Subject: Re: [PATCH v18 17/18] s390/Docs: new doc describing lock usage by the
 vfio_ap device driver



On 3/30/22 20:28, Halil Pasic wrote:
> On Mon, 14 Feb 2022 19:50:39 -0500
> Tony Krowiak <akrowiak@...ux.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@...ux.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/

I don't see the problem with instance, but I'll go ahead and make the 
change.

>
>> +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?

No, but it doesn't make the statement any less true. I'll take it out, 
however.

>
> (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.

How so? The vfio_ap_queue objects are linked to the ap_matrix_mdev object
to which the APQN is assigned. Other than that, they are contained in 
the driver
data of the queue device.

>
>> +
>> +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?

The matrix_dev->mdevs_lock controls access to all FIELDS contained within each matrix_mdev
and the matrix_dev->guests_lock controls access to matrix_mdev instances; in other words, the
matrix_dev->mdev_list and the matrix_mdev instances for the purposes further described
below.



>
>> +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.

I don't know how I can make it clearer. In 1 above, it states the it 
protects
access to the KVM pointer when it is being used to plug/unplug AP devices.
In other words, if the matrix_mdev->kvm pointer is being accessed just
to verify whether the mdev is attached to a guest or not, it is not 
necessary to
take the matrix_dev->guests_lock. On the other hand, whenever the 
matrix_mdev->kvm
pointer is being taken to dynamically update the guest's APCB (i.e., hot 
plug/unplug AP
devices), the matrix_dev->guests_lock must be held. Maybe if I had said 
hot plug/unplug
it would be clearer? I'm open to suggestions.

>
>> +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?

I've never actually encountered a deadlock condition while testing,
only a lockdep splat indicating a deadlock could occur. I'll go ahead
and add 'deadlock or lockdep splat'.

>
>> 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.

It may sound fishy, but it's a true statement. For example, there
are cases where the mdev is not attached to a KVM guest.
In that case, the matrix_mdev->kvm pointer will be NULL and
the matrix_mdev->kvm->lock will not be taken. Of course, the
matrix_dev->guests_lock will still have to be taken before the
matrix_dev->mdevs_lock. Maybe I should just make that point
clearer here.

After rereading the passage, I think that sentence should be
removed. It can be seen in the examples when the KVM lock
will not be taken.

>
>> 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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