Re: [Qemu-ppc] [PATCH] docs: provide documentation on the POWER9 XIVE interrupt controller

[Cédric Le Goater]

On 5/14/19 9:33 AM, Satheesh Rajendran wrote:
> On Tue, May 14, 2019 at 08:46:27AM +0200, Cédric Le Goater wrote:
>> 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 <address@hidden>
>> ---
>>  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
> 
> Overall doc, looks great, have few minor suggestions below.
>>
>> 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.
>> +
> can this be included?
> guest uses this device-tree entry(ibm,arch-vec-5-platform-support) to decide 
> on xive vs xics if dual set for ic-mode

yes. we can add some more information on the PAPR CAS negotiation process
as described by the PAPR specs. 

>> +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``
> 
> Some more explanations ``kernel_irqchip=off`` vs 
> ``kernel_irqchip=on``(default for full xive support) would help?
> kernel-irqchip=on - in-kernel accelerated one - more performance
> kernel-irqchip=off - fully emulated XIVE in QEMU - less performace

This is not XIVE specific, it's a QEMU level information which is
documented in the man page.
 
>> +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
> 
> Explanation on different ranges of ``LISN`` corresponds to 
> different type of interrupt sources,if applicable would help..

This is not specific to XIVE but, yes, we can add extra documentation  
file describing the sPAPR IRQ ranges.

>> +  range ``[ 0x0 ... 0x1FFF ]`` and its type : ``MSI`` or ``LSI``
> 
> Small explanation about `MSI` and `LSI` type interrupts and 
> example sources for each would help...

It does not belong in this file.

Thanks,

C.
 
>> +- 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: address@hidden
>>  S: Supported
>>  F: hw/*/*xive*
>>  F: include/hw/*/*xive*
>> +F: docs/ppc/xive.rst
>>  
>>  Subsystems
>>  ----------
>> -- 
>> 2.20.1
>>
>>
> 
> Regards,
> -Satheesh 
>