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

# RISC-V IOMMU support for RISC-V machines

QEMU implements a RISC-V IOMMU emulation based on the RISC-V IOMMU spec version 1.0 iommu1.0.

The emulation includes a PCI reference device, riscv-iommu-pci, that QEMU RISC-V boards can use. The ‘virt’ RISC-V machine is compatible with this device.

## riscv-iommu-pci reference device

This device implements the RISC-V IOMMU emulation as recommended by the section “Integrating an IOMMU as a PCIe device” of iommu1.0: a PCI device with base class 08h, sub-class 06h and programming interface 00h.

As a reference device it doesn’t implement anything outside of the specification, so it uses a generic default PCI ID given by QEMU: 1b36:0014.

To include the device in the ‘virt’ machine:

    $ qemu-system-riscv64 -M virt -device riscv-iommu-pci,[optional_pci_opts] (...)

This will add a RISC-V IOMMU PCI device in the board following any additional PCI parameters (like PCI bus address). The behavior of the RISC-V IOMMU is defined by the spec but its operation is OS dependent.

As of this writing the existing Linux kernel support linux-v8, not yet merged, does not have support for features like VFIO passthrough. The IOMMU emulation was tested using a public Ventana Micro Systems kernel repository in ventana-linux. This kernel is based on linux-v8 with additional patches that enable features like KVM VFIO passthrough with irqbypass. Until the kernel support is feature complete feel free to use the kernel available in the Ventana Micro Systems mirror.

The current Linux kernel support will use the IOMMU device to create IOMMU groups with any eligible cards available in the system, regardless of factors such as the order in which the devices are added in the command line.

This means that these command lines are equivalent as far as the current IOMMU kernel driver behaves:

    $ qemu-system-riscv64 \
          -M virt,aia=aplic-imsic,aia-guests=5 \
          -device riscv-iommu-pci,addr=1.0,vendor-id=0x1efd,device-id=0xedf1 \
          -device e1000e,netdev=net1 -netdev user,id=net1,net=192.168.0.0/24 \
          -device e1000e,netdev=net2 -netdev user,id=net2,net=192.168.200.0/24 \
          (...)

    $ qemu-system-riscv64 \
          -M virt,aia=aplic-imsic,aia-guests=5 \
          -device e1000e,netdev=net1 -netdev user,id=net1,net=192.168.0.0/24 \
          -device e1000e,netdev=net2 -netdev user,id=net2,net=192.168.200.0/24 \
          -device riscv-iommu-pci,addr=1.0,vendor-id=0x1efd,device-id=0xedf1 \
          (...)

Both will create iommu groups for the two e1000e cards.

Another thing to notice on linux-v8 and ventana-linux is that the kernel driver considers an IOMMU identified as a Rivos device, i.e. it uses Rivos vendor ID. To use the riscv-iommu-pci device with the existing kernel support we need to emulate a Rivos PCI IOMMU by setting ‘vendor-id’ and ‘device-id’:

    $ qemu-system-riscv64 -M virt \
       -device riscv-iommu-pci,vendor-id=0x1efd,device-id=0xedf1 (...)

Several options are available to control the capabilities of the device, namely:

- “bus”: the bus that the IOMMU device uses

- “ioatc-limit”: size of the Address Translation Cache (default to 2Mb)

- “intremap”: enable/disable MSI support

- “ats”: enable ATS support

- “off” (Out-of-reset translation mode: ‘on’ for DMA disabled, ‘off’ for ‘BARE’ (passthrough))

- “s-stage”: enable s-stage support

- “g-stage”: enable g-stage support