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