docs: update riscv/virt.rst with kernel-irqchip=split support

Also add a new page, docs/specs/riscv-aia.rst, where we're documenting
the state of AIA support in QEMU w.r.t the controllers being emulated or
not depending on the AIA and accelerator settings.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241119191706.718860-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/specs/index.rst

@@ -37,3 +37,4 @@ guest hardware that is specific to QEMU.
    rapl-msr
    rocker
    riscv-iommu
+   riscv-aia

docs/specs/riscv-aia.rst

@@ -0,0 +1,83 @@
+.. _riscv-aia:
+
+RISC-V AIA support for RISC-V machines
+======================================
+
+AIA (Advanced Interrupt Architecture) support is implemented in the ``virt``
+RISC-V machine for TCG and KVM accelerators.
+
+The support consists of two main modes:
+
+- "aia=aplic": adds one or more APLIC (Advanced Platform Level Interrupt Controller)
+  devices
+- "aia=aplic-imsic": adds one or more APLIC device and an IMSIC (Incoming MSI
+   Controller) device for each CPU
+
+From an user standpoint, these modes will behave the same regardless of the accelerator
+used.  From a developer standpoint the accelerator settings will change what it being
+emulated in userspace versus what is being emulated by an in-kernel irqchip.
+
+When running TCG, all controllers are emulated in userspace, including machine mode
+(m-mode) APLIC and IMSIC (when applicable).
+
+When running KVM:
+
+- no m-mode is provided, so there is no m-mode APLIC or IMSIC emulation regardless of
+  the AIA mode chosen
+- with "aia=aplic", s-mode APLIC will be emulated by userspace
+- with "aia=aplic-imsic" there are two possibilities.  If no additional KVM option
+  is provided there will be no APLIC or IMSIC emulation in userspace, and the virtual
+  machine will use the provided in-kernel APLIC and IMSIC controllers.  If the user
+  chooses to use the irqchip in split mode via "-accel kvm,kernel-irqchip=split",
+  s-mode APLIC will be emulated while using the s-mode IMSIC from the irqchip
+
+The following table summarizes how the AIA and accelerator options defines what
+we will emulate in userspace:
+
+
+.. list-table:: How AIA and accel options changes controller emulation
+   :widths: 25 25 25 25 25 25 25
+   :header-rows: 1
+
+   * - Accel
+     - Accel props
+     - AIA type
+     - APLIC m-mode
+     - IMSIC m-mode
+     - APLIC s-mode
+     - IMSIC s-mode
+   * - tcg
+     - ---
+     - aplic
+     - emul
+     - n/a
+     - emul
+     - n/a
+   * - tcg
+     - ---
+     - aplic-imsic
+     - emul
+     - emul
+     - emul
+     - emul
+   * - kvm
+     - ---
+     - aplic
+     - n/a
+     - n/a
+     - emul
+     - n/a
+   * - kvm
+     - none
+     - aplic-imsic
+     - n/a
+     - n/a
+     - in-kernel
+     - in-kernel
+   * - kvm
+     - irqchip=split
+     - aplic-imsic
+     - n/a
+     - n/a
+     - emul
+     - in-kernel