docs/specs: add riscv-iommu-sys information

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241106133407.604587-8-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/specs/riscv-iommu.rst

@@ -6,9 +6,9 @@ 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.
+The emulation includes a PCI reference device (riscv-iommu-pci) and a platform
+bus device (riscv-iommu-sys) that QEMU RISC-V boards can use.  The 'virt'
+RISC-V machine is compatible with both devices.
 
 riscv-iommu-pci reference device
 --------------------------------
@@ -83,6 +83,30 @@ Several options are available to control the capabilities of the device, namely:
 - "s-stage": enable s-stage support
 - "g-stage": enable g-stage support
 
+riscv-iommu-sys device
+----------------------
+
+This device implements the RISC-V IOMMU emulation as a platform bus device that
+RISC-V boards can use.
+
+For the 'virt' board the device is disabled by default.  To enable it use the
+'iommu-sys' machine option:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -M virt,iommu-sys=on (...)
+
+There is no options to configure the capabilities of this device in the 'virt'
+board using the QEMU command line.  The device is configured with the following
+riscv-iommu options:
+
+- "ioatc-limit": default value (2Mb)
+- "intremap": enabled
+- "ats": enabled
+- "off": on (DMA disabled)
+- "s-stage": enabled
+- "g-stage": enabled
+
 .. _iommu1.0: https://github.com/riscv-non-isa/riscv-iommu/releases/download/v1.0/riscv-iommu.pdf
 
 .. _linux-v8: https://lore.kernel.org/linux-riscv/cover.1718388908.git.tjeznach@rivosinc.com/