[Qemu-devel] [PATCH] docs: updates on the POWER9 XIVE interrupt controller documentation

["Cédric Le Goater" <clg@kaod.org>]

This includes various small updates and a better description of the
chosen interrupt mode resulting from the combination of the 'ic-mode'
machine option, the 'kernel_irqchip' option, guest support and KVM
support.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---

 David, please check the tables below. Thanks

 docs/specs/ppc-spapr-xive.rst | 112 ++++++++++++++++++++++++++++++++--
 docs/specs/ppc-xive.rst       |   9 +--
 2 files changed, 111 insertions(+), 10 deletions(-)

diff --git a/docs/specs/ppc-spapr-xive.rst b/docs/specs/ppc-spapr-xive.rst
index 539ce7ca4e90..7a64c9d04951 100644
--- a/docs/specs/ppc-spapr-xive.rst
+++ b/docs/specs/ppc-spapr-xive.rst
@@ -34,19 +34,118 @@ CAS Negotiation
 ---------------
 
 QEMU advertises the supported interrupt modes in the device tree
-property "ibm,arch-vec-5-platform-support" in byte 23 and the OS
-Selection for XIVE is indicated in the "ibm,architecture-vec-5"
+property ``ibm,arch-vec-5-platform-support`` in byte 23 and the OS
+Selection for XIVE is indicated in the ``ibm,architecture-vec-5``
 property byte 23.
 
 The interrupt modes supported by the machine depend on the CPU type
 (POWER9 is required for XIVE) but also on the machine property
 ``ic-mode`` which can be set on the command line. It can take the
-following values: ``xics``, ``xive``, ``dual`` and currently ``xics``
-is the default but it may change in the future.
+following values: ``xics``, ``xive``, and ``dual`` which is the
+default mode. ``dual`` means that both modes XICS **and** XIVE are
+supported and if the guest OS supports XIVE, this mode will be
+selected.
 
 The choosen interrupt mode is activated after a reconfiguration done
 in a machine reset.
 
+KVM negotiation
+---------------
+
+When the guest starts under KVM, the capabilities of the host kernel
+and QEMU are also negotiated. Depending on the version of the host
+kernel, KVM will advertise the XIVE capability to QEMU or not.
+
+Nevertheless, the available interrupt modes in the machine should not
+depend on the XIVE KVM capability of the host. On older kernels
+without XIVE KVM support, QEMU will use the emulated XIVE device as a
+fallback and on newer kernels (>=5.2), the KVM XIVE device.
+
+As a final refinement, the user can also switch the use of the KVM
+device with the machine option ``kernel_irqchip``.
+
+
+XIVE support in KVM
+~~~~~~~~~~~~~~~~~~~
+
+For guest OSes supporting XIVE, the resulting interrupt modes on host
+kernels with XIVE KVM support are the following:
+
+==============  =============  =============  ================
+ic-mode                            kernel_irqchip
+--------------  ----------------------------------------------
+/               allowed        off            on
+                (default)
+==============  =============  =============  ================
+dual (default)  XIVE KVM       XIVE emul.     XIVE KVM
+xive            XIVE KVM       XIVE emul.     XIVE KVM
+xics            XICS KVM       XICS emul.     XICS KVM
+==============  =============  =============  ================
+
+For legacy guest OSes without XIVE support, the resulting interrupt
+modes are the following:
+
+==============  =============  =============  ================
+ic-mode                            kernel_irqchip
+--------------  ----------------------------------------------
+/               allowed        off            on
+                (default)
+==============  =============  =============  ================
+dual (default)  XICS KVM       XICS emul.     XICS KVM
+xive            QEMU error(3)  QEMU error(3)  QEMU error(3)
+xics            XICS KVM       XICS emul.     XICS KVM
+==============  =============  =============  ================
+
+(3) QEMU fails at CAS with ``Guest requested unavailable interrupt
+    mode (XICS), either don't set the ic-mode machine property or try
+    ic-mode=xics or ic-mode=dual``
+
+
+No XIVE support in KVM
+~~~~~~~~~~~~~~~~~~~~~~
+
+For guest OSes supporting XIVE, the resulting interrupt modes on host
+kernels without XIVE KVM support are the following:
+
+==============  =============  =============  ================
+ic-mode                            kernel_irqchip
+--------------  ----------------------------------------------
+/               allowed        off            on
+                (default)
+==============  =============  =============  ================
+dual (default)  XIVE emul.(1)  XIVE emul.     QEMU error (2)
+xive            XIVE emul.(1)  XIVE emul.     QEMU error (2)
+xics            XICS KVM       XICS emul.     XICS KVM
+==============  =============  =============  ================
+
+
+(1) QEMU warns with ``warning: kernel_irqchip requested but unavailable:
+    IRQ_XIVE capability must be present for KVM``
+(2) QEMU fails with ``kernel_irqchip requested but unavailable:
+    IRQ_XIVE capability must be present for KVM``
+
+
+For legacy guest OSes without XIVE support, the resulting interrupt
+modes are the following:
+
+==============  =============  =============  ================
+ic-mode                            kernel_irqchip
+--------------  ----------------------------------------------
+/               allowed        off            on
+                (default)
+==============  =============  =============  ================
+dual (default)  QEMU error(4)  XICS emul.     QEMU error(4)
+xive            QEMU error(3)  QEMU error(3)  QEMU error(3)
+xics            XICS KVM       XICS emul.     XICS KVM
+==============  =============  =============  ================
+
+(3) QEMU fails at CAS with ``Guest requested unavailable interrupt
+    mode (XICS), either don't set the ic-mode machine property or try
+    ic-mode=xics or ic-mode=dual``
+(4) QEMU/KVM incompatibility due to device destruction in reset. This
+    needs to be addressed more cleanly with an error.
+
+
 XIVE Device tree properties
 ---------------------------
 
@@ -92,10 +191,11 @@ for both interrupt mode. The different ranges are defined as follow :
 - ``0x0000 .. 0x0FFF`` 4K CPU IPIs (only used under XIVE)
 - ``0x1000 .. 0x1000`` 1 EPOW
 - ``0x1001 .. 0x1001`` 1 HOTPLUG
+- ``0x1002 .. 0x10FF`` unused
 - ``0x1100 .. 0x11FF`` 256 VIO devices
-- ``0x1200 .. 0x127F`` 32 PHBs devices
+- ``0x1200 .. 0x127F`` 32x4 LSIs for PHB devices
 - ``0x1280 .. 0x12FF`` unused
-- ``0x1300 .. 0x1FFF`` PHB MSIs
+- ``0x1300 .. 0x1FFF`` PHB MSIs (dynamically allocated)
 
 Monitoring XIVE
 ---------------
diff --git a/docs/specs/ppc-xive.rst b/docs/specs/ppc-xive.rst
index b997dc062910..148d57eb6ab2 100644
--- a/docs/specs/ppc-xive.rst
+++ b/docs/specs/ppc-xive.rst
@@ -20,10 +20,11 @@ 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.
+  (SC). These are found in PCI PHBs, in the Processor Service
+  Interface (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
-- 
2.21.0