FLIC (floating interrupt controller)
====================================

FLIC handles floating (non per-cpu) interrupts, i.e. I/O, service and some
machine check interruptions. All interrupts are stored in a per-vm list of
pending interrupts. FLIC performs operations on this list.

Only one FLIC instance may be instantiated.

FLIC provides support to
- add interrupts (KVM_DEV_FLIC_ENQUEUE)
- inspect currently pending interrupts (KVM_FLIC_GET_ALL_IRQS)
- purge all pending floating interrupts (KVM_DEV_FLIC_CLEAR_IRQS)
- purge one pending floating I/O interrupt (KVM_DEV_FLIC_CLEAR_IO_IRQ)
- enable/disable for the guest transparent async page faults
- register and modify adapter interrupt sources (KVM_DEV_FLIC_ADAPTER_*)

Groups:
  KVM_DEV_FLIC_ENQUEUE
    Passes a buffer and length into the kernel which are then injected into
    the list of pending interrupts.
    attr->addr contains the pointer to the buffer and attr->attr contains
    the length of the buffer.
    The format of the data structure kvm_s390_irq as it is copied from userspace
    is defined in usr/include/linux/kvm.h.

  KVM_DEV_FLIC_GET_ALL_IRQS
    Copies all floating interrupts into a buffer provided by userspace.
    When the buffer is too small it returns -ENOMEM, which is the indication
    for userspace to try again with a bigger buffer.
    -ENOBUFS is returned when the allocation of a kernelspace buffer has
    failed.
    -EFAULT is returned when copying data to userspace failed.
    All interrupts remain pending, i.e. are not deleted from the list of
    currently pending interrupts.
    attr->addr contains the userspace address of the buffer into which all
    interrupt data will be copied.
    attr->attr contains the size of the buffer in bytes.

  KVM_DEV_FLIC_CLEAR_IRQS
    Simply deletes all elements from the list of currently pending floating
    interrupts.  No interrupts are injected into the guest.

  KVM_DEV_FLIC_CLEAR_IO_IRQ
    Deletes one (if any) I/O interrupt for a subchannel identified by the
    subsystem identification word passed via the buffer specified by
    attr->addr (address) and attr->attr (length).

  KVM_DEV_FLIC_APF_ENABLE
    Enables async page faults for the guest. So in case of a major page fault
    the host is allowed to handle this async and continues the guest.

  KVM_DEV_FLIC_APF_DISABLE_WAIT
    Disables async page faults for the guest and waits until already pending
    async page faults are done. This is necessary to trigger a completion interrupt
    for every init interrupt before migrating the interrupt list.

  KVM_DEV_FLIC_ADAPTER_REGISTER
    Register an I/O adapter interrupt source. Takes a kvm_s390_io_adapter
    describing the adapter to register:

struct kvm_s390_io_adapter {
	__u32 id;
	__u8 isc;
	__u8 maskable;
	__u8 swap;
	__u8 flags;
};

   id contains the unique id for the adapter, isc the I/O interruption subclass
   to use, maskable whether this adapter may be masked (interrupts turned off),
   swap whether the indicators need to be byte swapped, and flags contains
   further characteristics of the adapter.
   Currently defined values for 'flags' are:
   - KVM_S390_ADAPTER_SUPPRESSIBLE: adapter is subject to AIS
     (adapter-interrupt-suppression) facility. This flag only has an effect if
     the AIS capability is enabled.
   Unknown flag values are ignored.


  KVM_DEV_FLIC_ADAPTER_MODIFY
    Modifies attributes of an existing I/O adapter interrupt source. Takes
    a kvm_s390_io_adapter_req specifying the adapter and the operation:

struct kvm_s390_io_adapter_req {
	__u32 id;
	__u8 type;
	__u8 mask;
	__u16 pad0;
	__u64 addr;
};

    id specifies the adapter and type the operation. The supported operations
    are:

    KVM_S390_IO_ADAPTER_MASK
      mask or unmask the adapter, as specified in mask

    KVM_S390_IO_ADAPTER_MAP
      perform a gmap translation for the guest address provided in addr,
      pin a userspace page for the translated address and add it to the
      list of mappings
      Note: A new mapping will be created unconditionally; therefore,
            the calling code should avoid making duplicate mappings.

    KVM_S390_IO_ADAPTER_UNMAP
      release a userspace page for the translated address specified in addr
      from the list of mappings

Note: The KVM_SET_DEVICE_ATTR/KVM_GET_DEVICE_ATTR device ioctls executed on
FLIC with an unknown group or attribute gives the error code EINVAL (instead of
ENXIO, as specified in the API documentation). It is not possible to conclude
that a FLIC operation is unavailable based on the error code resulting from a
usage attempt.