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docs: provide documentation on the POWER9 XIVE interrupt controller v1 PROPOSED

Cédric Le Goater
Cédric Le Goater: 1
 docs: provide documentation on the POWER9 XIVE interrupt controller

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[Qemu-devel] [PATCH] docs: provide documentation on the POWER9 XIVE interrupt controller Export this patch

Cédric Le Goater
This documents the overall XIVE architecture and gives an overview of
the QEMU models. It also provides documentation on the 'info pic'
command.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 docs/index.rst     |   1 +
 docs/ppc/index.rst |  13 ++
 docs/ppc/xive.rst  | 344 +++++++++++++++++++++++++++++++++++++++++++++
 MAINTAINERS        |   1 +
 4 files changed, 359 insertions(+)
 create mode 100644 docs/ppc/index.rst
 create mode 100644 docs/ppc/xive.rst
diff --git a/docs/index.rst b/docs/index.rst
index 3690955dd1f5..557fe86233e3 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -12,4 +12,5 @@ Welcome to QEMU's documentation!

   interop/index
   devel/index
   ppc/index

diff --git a/docs/ppc/index.rst b/docs/ppc/index.rst
new file mode 100644
index 000000000000..146f416ea3a0
--- /dev/null
+++ b/docs/ppc/index.rst
@@ -0,0 +1,13 @@
.. This is the top level page for the 'ppc' manual


QEMU PowerPC Machine and Controller Guide
=========================================


Contents:

.. toctree::
   :maxdepth: 2

   xive
diff --git a/docs/ppc/xive.rst b/docs/ppc/xive.rst
new file mode 100644
index 000000000000..90ddde6bf39f
--- /dev/null
+++ b/docs/ppc/xive.rst
@@ -0,0 +1,344 @@
================================
POWER9 XIVE interrupt controller
================================

The POWER9 processor comes with a new interrupt controller
architecture, called XIVE as "eXternal Interrupt Virtualization
Engine".

Compared to the previous architecture, the main characteristics of
XIVE are to support a larger number of interrupt sources and to
deliver interrupts directly to virtual processors without hypervisor
assistance. This removes the context switches required for the
delivery process.


Overall architecture
====================

The XIVE IC is composed of three sub-engines, each taking care of a
processing layer of external interrupts:

- Interrupt Virtualization Source Engine (IVSE), or Source Controller
  (SC). These are found in PCI PHBs, in the PSI host bridge
  controller, but also inside the main controller for the core IPIs
  and other sub-chips (NX, CAP, NPU) of the chip/processor. They are
  configured to feed the IVRE with events.
- Interrupt Virtualization Routing Engine (IVRE) or Virtualization
  Controller (VC). It handles event coalescing and perform interrupt
  routing by matching an event source number with an Event
  Notification Descriptor (END).
- Interrupt Virtualization Presentation Engine (IVPE) or Presentation
  Controller (PC). It maintains the interrupt context state of each
  thread and handles the delivery of the external interrupt to the
  thread.

::

                XIVE Interrupt Controller
                +------------------------------------+      IPIs
                | +---------+ +---------+ +--------+ |    +-------+
                | |IVRE     | |Common Q | |IVPE    |----> | CORES |
                | |     esb | |         | |        |----> |       |
                | |     eas | |  Bridge | |   tctx |----> |       |
                | |SC   end | |         | |    nvt | |    |       |
    +------+    | +---------+ +----+----+ +--------+ |    +-+-+-+-+
    | RAM  |    +------------------|-----------------+      | | |
    |      |                       |                        | | |
    |      |                       |                        | | |
    |      |  +--------------------v------------------------v-v-v--+    other
    |      <--+                     Power Bus                      +--> chips
    |  esb |  +---------+-----------------------+------------------+
    |  eas |            |                       |
    |  end |         +--|------+                |
    |  nvt |       +----+----+ |           +----+----+
    +------+       |IVSE     | |           |IVSE     |
                   |         | |           |         |
                   | PQ-bits | |           | PQ-bits |
                   | local   |-+           |  in VC  |
                   +---------+             +---------+
                      PCIe                 NX,NPU,CAPI


    PQ-bits: 2 bits source state machine (P:pending Q:queued)
    esb: Event State Buffer (Array of PQ bits in an IVSE)
    eas: Event Assignment Structure
    end: Event Notification Descriptor
    nvt: Notification Virtual Target
    tctx: Thread interrupt Context registers



XIVE internal tables
--------------------

Each of the sub-engines uses a set of tables to redirect interrupts
from event sources to CPU threads.

::

                                            +-------+
    User or O/S                             |  EQ   |
        or                          +------>|entries|
    Hypervisor                      |       |  ..   |
      Memory                        |       +-------+
                                    |           ^
                                    |           |
               +-------------------------------------------------+
                                    |           |
    Hypervisor      +------+    +---+--+    +---+--+   +------+
      Memory        | ESB  |    | EAT  |    | ENDT |   | NVTT |
     (skiboot)      +----+-+    +----+-+    +----+-+   +------+
                      ^  |        ^  |        ^  |       ^
                      |  |        |  |        |  |       |
               +-------------------------------------------------+
                      |  |        |  |        |  |       |
                      |  |        |  |        |  |       |
                 +----|--|--------|--|--------|--|-+   +-|-----+    +------+
                 |    |  |        |  |        |  | |   | | tctx|    |Thread|
     IPI or   ---+    +  v        +  v        +  v |---| +  .. |----->     |
    HW events    |                                 |   |       |    |      |
                 |             IVRE                |   | IVPE  |    +------+
                 +---------------------------------+   +-------+


The IVSE have a 2-bits state machine, P for pending and Q for queued,
for each source that allows events to be triggered. They are stored in
an Event State Buffer (ESB) array and can be controlled by MMIOs.

If the event is let through, the IVRE looks up in the Event Assignment
Structure (EAS) table for an Event Notification Descriptor (END)
configured for the source. Each Event Notification Descriptor defines
a notification path to a CPU and an in-memory Event Queue, in which
will be enqueued an EQ data for the O/S to pull.

The IVPE determines if a Notification Virtual Target (NVT) can handle
the event by scanning the thread contexts of the VCPUs dispatched on
the processor HW threads. It maintains the interrupt context state of
each thread in a NVT table.

XIVE thread interrupt context
-----------------------------

The XIVE presenter can generate four different exceptions to its
HW threads:

- hypervisor exception
- O/S exception
- Event-Based Branch (user level)
- msgsnd (doorbell)

Each exception has a state independent from the others called a Thread
Interrupt Management context. This context is a set of registers which
lets the thread handle priority management and interrupt
acknowledgment among other things. The most important ones being :

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

TIMA
~~~~

The Thread Interrupt Management registers are accessible through a
specific MMIO region, called the Thread Interrupt Management Area
(TIMA), four aligned pages, each exposing a different view of the
registers. First page (page address ending in ``0b00``) gives access
to the entire context and is reserved for the ring 0 view for the
physical thread context. The second (page address ending in ``0b01``)
is for the hypervisor, ring 1 view. The third (page address ending in
``0b10``) is for the operating system, ring 2 view. The fourth (page
address ending in ``0b11``) is for user level, ring 3 view.

Interrupt flow from an O/S perspective
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After an event data has been enqueued in the O/S Event Queue, the IVPE
raises the bit corresponding to the priority of the pending interrupt
in the register IBP (Interrupt Pending Buffer) to indicate that an
event is pending in one of the 8 priority queues. The Pending
Interrupt Priority Register (PIPR) is also updated using the IPB. This
register represent the priority of the most favored pending
notification.

The PIPR is then compared to the the Current Processor Priority
Register (CPPR). If it is more favored (numerically less than), the
CPU interrupt line is raised and the EO bit of the Notification Source
Register (NSR) is updated to notify the presence of an exception for
the O/S. The O/S acknowledges the interrupt with a special load in the
Thread Interrupt Management Area.

The O/S handles the interrupt and when done, performs an EOI using a
MMIO operation on the ESB management page of the associate source.


Overview of the QEMU models for XIVE
====================================

The XiveSource models the IVSE in general, internal and external. It
handles the source ESBs and the MMIO interface to control them.

The XiveNotifier is a small helper interface interconnecting the
XiveSource to the XiveRouter.

The XiveRouter is an abstract model acting as a combined IVRE and
IVPE. It routes event notifications using the EAS and END tables to
the IVPE sub-engine which does a CAM scan to find a CPU to deliver the
exception. Storage should be provided by the inheriting classes.

XiveEnDSource is a special source object. It exposes the END ESB MMIOs
of the Event Queues which are used for coalescing event notifications
and for escalation. Not used on the field, only to sync the EQ cache
in OPAL.

Finally, the XiveTCTX contains the interrupt state context of a thread,
four sets of registers, one for each exception that can be delivered
to a CPU. These contexts are scanned by the IVPE to find a matching VP
when a notification is triggered. It also models the Thread Interrupt
Management Area (TIMA), which exposes the thread context registers to
the CPU for interrupt management.


XIVE for sPAPR (pseries machines)
=================================

SpaprXive models the XIVE interrupt controller of a ``pseries``
machine. It inherits from the XiveRouter and provisions storage for
the EAS and END tables. The NVT table does not need a backend in
sPAPR. It owns a XiveSource object for the IPIs and the virtual device
interrupts, a memory region for the TIMA and a XiveENDSource object to
manage the END ESBs (not used by Linux).

These choices were made to have a sPAPR interrupt controller consistent
with the one found on baremetal and to facilitate KVM support, the
main difficulty being the host memory regions exposed to the guest.

CAS Negotiation
---------------

The interrupt mode advertised by the ``pseries`` machine in the CAS
negotiation process depends on the CPU type (XIVE requires POWER9) but
also on the machine property ``ic-mode`` which can take the following
values: ``xics``, ``xive`` and ``dual``. ``xics`` is currently the
default mode but it should change in the future.

The choosen interrupt mode is activated after a reconfiguration done
in a machine reset.

KVM support
-----------

Two host memory regions are exposed to the guest and require special
attention at initialization :

- ESB MMIOs
- Thread Interrupt Management Area (TIMA)

When using the KVM device, these are `ram device` memory mappings,
similarly to VFIO, exposed to the guest and the associated VMAs on the
host are populated dynamically with the appropriate pages using a
fault handler.

The models uses KVM accessors to synchronize the QEMU state with KVM :

- the source configuration (EAT)
- the END configuration (ENDT)
- the O/S EQ state (toggle bit and index)
- the thread interrupt context registers.

Hybrid guest using KVM and an emulated irqchip ``kernel_irqchip=off``
is supported.

Monitoring XIVE
---------------

The state of the XIVE interrupt controller can be queried through the
monitor commands ``info pic``. The output comes in two parts.

First, the state of the thread interrupt context registers is dumped
for each CPU :

::

   (qemu) info pic
   CPU[0000]:   QW   NSR CPPR IPB LSMFB ACK# INC AGE PIPR  W2
   CPU[0000]: USER    00   00  00    00   00  00  00   00  00000000
   CPU[0000]:   OS    00   ff  00    00   ff  00  ff   ff  80000400
   CPU[0000]: POOL    00   00  00    00   00  00  00   00  00000000
   CPU[0000]: PHYS    00   00  00    00   00  00  00   ff  00000000
   ...

In the case of a ``pseries`` machine, QEMU acts as the hypervisor and only
the O/S and USER register rings make sense. ``W2`` contains the vCPU CAM
line which is set to the VP identifier.

Then comes the routing information which aggregates the EAS and the
END configuration:

::

   ...
   LISN         PQ    EISN     CPU/PRIO EQ
   00000000 MSI --    00000010   0/6    380/16384 @1fe3e0000 ^1 [ 80000010 ... ]
   00000001 MSI --    00000010   1/6    305/16384 @1fc230000 ^1 [ 80000010 ... ]
   00000002 MSI --    00000010   2/6    220/16384 @1fc2f0000 ^1 [ 80000010 ... ]
   00000003 MSI --    00000010   3/6    201/16384 @1fc390000 ^1 [ 80000010 ... ]
   00000004 MSI -Q  M 00000000
   00000005 MSI -Q  M 00000000
   00000006 MSI -Q  M 00000000
   00000007 MSI -Q  M 00000000
   00001000 MSI --    00000012   0/6    380/16384 @1fe3e0000 ^1 [ 80000010 ... ]
   00001001 MSI --    00000013   0/6    380/16384 @1fe3e0000 ^1 [ 80000010 ... ]
   00001100 MSI --    00000100   1/6    305/16384 @1fc230000 ^1 [ 80000010 ... ]
   00001101 MSI -Q  M 00000000
   00001200 LSI -Q  M 00000000
   00001201 LSI -Q  M 00000000
   00001202 LSI -Q  M 00000000
   00001203 LSI -Q  M 00000000
   00001300 MSI --    00000102   1/6    305/16384 @1fc230000 ^1 [ 80000010 ... ]
   00001301 MSI --    00000103   2/6    220/16384 @1fc2f0000 ^1 [ 80000010 ... ]
   00001302 MSI --    00000104   3/6    201/16384 @1fc390000 ^1 [ 80000010 ... ]

The source information and configuration:

- The ``LISN`` column outputs the interrupt number of the source in
  range ``[ 0x0 ... 0x1FFF ]`` and its type : ``MSI`` or ``LSI``
- The ``PQ`` column reflects the state of the PQ bits of the source :

  - ``--`` source is ready to take events
  - ``P-`` an event was sent and an EOI is PENDING
  - ``PQ`` an event was QUEUED
  - ``-Q`` source is OFF

  a ``M`` indicates that source is *MASKED* at the EAS level,

The targeting configuration :

- The ``EISN`` column is the event data what will be queued in the event
  queue of the O/S.
- The ``CPU/PRIO`` column is the tuple defining the CPU number and
  priority queue serving the source.
- The ``EQ`` column outputs :

  - the current index of the event queue/ the max number of entries
  - the O/S event queue address
  - the toggle bit
  - the last entries that were pushed in the event queue.



XIVE for PowerNV
================

The PnvXIVE model uses the XiveRouter abstract model just like
sPAPRXive. It provides accessors to the EAS, END and NVT tables which
are stored in the QEMU PowerNV machine and not in QEMU anymore. It
owns a set of memory regions for the IC registers, the ESBs, the END
ESBs, the TIMA, the notification MMIO.

Multichip is supported and the available IVSEs are the internal one
for the IPIS, the PSI host bridge controller and PHB4.

The next interesting step would be to add escalation events and model
the VCPU dispatching to support emulated KVM guests.
diff --git a/MAINTAINERS b/MAINTAINERS
index 66ddbda9c958..a896c7407294 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1697,6 +1697,7 @@ L: qemu-ppc@nongnu.org
S: Supported
F: hw/*/*xive*
F: include/hw/*/*xive*
F: docs/ppc/xive.rst

Subsystems
----------
-- 
2.20.1



      
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