RISC-V AIA support for RISC-V machines — QEMU documentation

# 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:

How AIA and accel options changes controller emulation
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

## KVM accel option ‘riscv-aia’

The KVM accelerator property ‘riscv-aia’ interacts with the “aia=aplic-imsic” to determine how the host KVM module will provide the in-kernel IMSIC s-mode controller. The ‘kernel-irqchip’ setting has no impact in ‘riscv-aia’ given that any available ‘kernel-irqchip’ setting will always have an in-kernel IMSIC s-mode. ‘riscv-aia’ has no impact in APLIC m-mode/s-mode and IMSIC m-mode settings.

How ‘riscv-aia’ changes in-kernel IMSIC s-mode provisioning
Accel | KVM riscv-aia | AIA type | IMSIC s-mode
--- | --- | --- | ---
kvm | none | aplic-imsic | in-kernel, default to ‘auto’
kvm | auto | aplic-imsic | in-kernel, hwaccel if available, emul otherwise
kvm | hwaccel | aplic-imsic | in-kernel, use IMSIC controller from guest hardware
kvm | emul | aplic-imsic | in-kernel, IMSIC is emulated by KVM