POWER9 eXternal Interrupt Virtualization Engine (XIVE Gen1)
==========================================================

Device types supported:
  KVM_DEV_TYPE_XIVE     POWER9 XIVE Interrupt Controller generation 1

This device acts as a VM interrupt controller. It provides the KVM
interface to configure the interrupt sources of a VM in the underlying
POWER9 XIVE interrupt controller.

Only one XIVE instance may be instantiated. A guest XIVE device
requires a POWER9 host and the guest OS should have support for the
XIVE native exploitation interrupt mode. If not, it should run using
the legacy interrupt mode, referred as XICS (POWER7/8).

* Device Mappings

  The KVM device exposes different MMIO ranges of the XIVE HW which
  are required for interrupt management. These are exposed to the
  guest in VMAs populated with a custom VM fault handler.

  1. Thread Interrupt Management Area (TIMA)

  Each thread has an associated Thread Interrupt Management context
  composed of a set of registers. These registers let the thread
  handle priority management and interrupt acknowledgment. The most
  important are :

      - Interrupt Pending Buffer     (IPB)
      - Current Processor Priority   (CPPR)
      - Notification Source Register (NSR)

  They are exposed to software in four different pages each proposing
  a view with a different privilege. The first page is for the
  physical thread context and the second for the hypervisor. Only the
  third (operating system) and the fourth (user level) are exposed the
  guest.

  2. Event State Buffer (ESB)

  Each source is associated with an Event State Buffer (ESB) with
  either a pair of even/odd pair of pages which provides commands to
  manage the source: to trigger, to EOI, to turn off the source for
  instance.

  3. Device pass-through

  When a device is passed-through into the guest, the source
  interrupts are from a different HW controller (PHB4) and the ESB
  pages exposed to the guest should accommadate this change.

  The passthru_irq helpers, kvmppc_xive_set_mapped() and
  kvmppc_xive_clr_mapped() are called when the device HW irqs are
  mapped into or unmapped from the guest IRQ number space. The KVM
  device extends these helpers to clear the ESB pages of the guest IRQ
  number being mapped and then lets the VM fault handler repopulate.
  The handler will insert the ESB page corresponding to the HW
  interrupt of the device being passed-through or the initial IPI ESB
  page if the device has being removed.

  The ESB remapping is fully transparent to the guest and the OS
  device driver. All handling is done within VFIO and the above
  helpers in KVM-PPC.

* Groups:

  1. KVM_DEV_XIVE_GRP_CTRL
  Provides global controls on the device
  Attributes:
    1.1 KVM_DEV_XIVE_RESET (write only)
    Resets the interrupt controller configuration for sources and event
    queues. To be used by kexec and kdump.
    Errors: none

    1.2 KVM_DEV_XIVE_EQ_SYNC (write only)
    Sync all the sources and queues and mark the EQ pages dirty. This
    to make sure that a consistent memory state is captured when
    migrating the VM.
    Errors: none

    1.3 KVM_DEV_XIVE_NR_SERVERS (write only)
    The kvm_device_attr.addr points to a __u32 value which is the number of
    interrupt server numbers (ie, highest possible vcpu id plus one).
    Errors:
      -EINVAL: Value greater than KVM_MAX_VCPU_ID.
      -EFAULT: Invalid user pointer for attr->addr.
      -EBUSY:  A vCPU is already connected to the device.

  2. KVM_DEV_XIVE_GRP_SOURCE (write only)
  Initializes a new source in the XIVE device and mask it.
  Attributes:
    Interrupt source number  (64-bit)
  The kvm_device_attr.addr points to a __u64 value:
  bits:     | 63   ....  2 |   1   |   0
  values:   |    unused    | level | type
  - type:  0:MSI 1:LSI
  - level: assertion level in case of an LSI.
  Errors:
    -E2BIG:  Interrupt source number is out of range
    -ENOMEM: Could not create a new source block
    -EFAULT: Invalid user pointer for attr->addr.
    -ENXIO:  Could not allocate underlying HW interrupt

  3. KVM_DEV_XIVE_GRP_SOURCE_CONFIG (write only)
  Configures source targeting
  Attributes:
    Interrupt source number  (64-bit)
  The kvm_device_attr.addr points to a __u64 value:
  bits:     | 63   ....  33 |  32  | 31 .. 3 |  2 .. 0
  values:   |    eisn       | mask |  server | priority
  - priority: 0-7 interrupt priority level
  - server: CPU number chosen to handle the interrupt
  - mask: mask flag (unused)
  - eisn: Effective Interrupt Source Number
  Errors:
    -ENOENT: Unknown source number
    -EINVAL: Not initialized source number
    -EINVAL: Invalid priority
    -EINVAL: Invalid CPU number.
    -EFAULT: Invalid user pointer for attr->addr.
    -ENXIO:  CPU event queues not configured or configuration of the
             underlying HW interrupt failed
    -EBUSY:  No CPU available to serve interrupt

  4. KVM_DEV_XIVE_GRP_EQ_CONFIG (read-write)
  Configures an event queue of a CPU
  Attributes:
    EQ descriptor identifier (64-bit)
  The EQ descriptor identifier is a tuple (server, priority) :
  bits:     | 63   ....  32 | 31 .. 3 |  2 .. 0
  values:   |    unused     |  server | priority
  The kvm_device_attr.addr points to :
    struct kvm_ppc_xive_eq {
	__u32 flags;
	__u32 qshift;
	__u64 qaddr;
	__u32 qtoggle;
	__u32 qindex;
	__u8  pad[40];
    };
  - flags: queue flags
    KVM_XIVE_EQ_ALWAYS_NOTIFY (required)
	forces notification without using the coalescing mechanism
	provided by the XIVE END ESBs.
  - qshift: queue size (power of 2)
  - qaddr: real address of queue
  - qtoggle: current queue toggle bit
  - qindex: current queue index
  - pad: reserved for future use
  Errors:
    -ENOENT: Invalid CPU number
    -EINVAL: Invalid priority
    -EINVAL: Invalid flags
    -EINVAL: Invalid queue size
    -EINVAL: Invalid queue address
    -EFAULT: Invalid user pointer for attr->addr.
    -EIO:    Configuration of the underlying HW failed

  5. KVM_DEV_XIVE_GRP_SOURCE_SYNC (write only)
  Synchronize the source to flush event notifications
  Attributes:
    Interrupt source number  (64-bit)
  Errors:
    -ENOENT: Unknown source number
    -EINVAL: Not initialized source number

* VCPU state

  The XIVE IC maintains VP interrupt state in an internal structure
  called the NVT. When a VP is not dispatched on a HW processor
  thread, this structure can be updated by HW if the VP is the target
  of an event notification.

  It is important for migration to capture the cached IPB from the NVT
  as it synthesizes the priorities of the pending interrupts. We
  capture a bit more to report debug information.

  KVM_REG_PPC_VP_STATE (2 * 64bits)
  bits:     |  63  ....  32  |  31  ....  0  |
  values:   |   TIMA word0   |   TIMA word1  |
  bits:     | 127       ..........       64  |
  values:   |            unused              |

* Migration:

  Saving the state of a VM using the XIVE native exploitation mode
  should follow a specific sequence. When the VM is stopped :

  1. Mask all sources (PQ=01) to stop the flow of events.

  2. Sync the XIVE device with the KVM control KVM_DEV_XIVE_EQ_SYNC to
  flush any in-flight event notification and to stabilize the EQs. At
  this stage, the EQ pages are marked dirty to make sure they are
  transferred in the migration sequence.

  3. Capture the state of the source targeting, the EQs configuration
  and the state of thread interrupt context registers.

  Restore is similar :

  1. Restore the EQ configuration. As targeting depends on it.
  2. Restore targeting
  3. Restore the thread interrupt contexts
  4. Restore the source states
  5. Let the vCPU run