[PATCH v4 14/14] docs/specs: add riscv-iommu

[Daniel Henrique Barboza]

Add a simple guideline to use the existing RISC-V IOMMU support we just
added.

This doc will be updated once we add the riscv-iommu-sys device.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 docs/specs/index.rst       |  1 +
 docs/specs/riscv-iommu.rst | 55 ++++++++++++++++++++++++++++++++++++++
 docs/system/riscv/virt.rst | 13 +++++++++
 3 files changed, 69 insertions(+)
 create mode 100644 docs/specs/riscv-iommu.rst

diff --git a/docs/specs/index.rst b/docs/specs/index.rst
index 1484e3e760..c68cd9ae6c 100644
--- a/docs/specs/index.rst
+++ b/docs/specs/index.rst
@@ -33,3 +33,4 @@ guest hardware that is specific to QEMU.
    virt-ctlr
    vmcoreinfo
    vmgenid
+   riscv-iommu
diff --git a/docs/specs/riscv-iommu.rst b/docs/specs/riscv-iommu.rst
new file mode 100644
index 0000000000..fa38ff7667
--- /dev/null
+++ b/docs/specs/riscv-iommu.rst
@@ -0,0 +1,55 @@
+.. _riscv-iommu:
+
+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 [1].
+
+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.
+
+A platform device that implements the RISC-V IOMMU will be added in the
+future.
+
+
+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 [1]: 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:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -M virt -device riscv-iommu-pci (...)
+
+As of this writing the existing Linux kernel support [2], not yet merged, is 
being
+created 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':
+
+.. code-block:: bash
+
+  $ 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"
+- "ioatc-limit"
+- "intremap"
+- "ats"
+- "off" (Out-of-reset translation mode: 'on' for DMA disabled, 'off' for 
'BARE' (passthrough))
+- "s-stage"
+- "g-stage"
+
+
+[1] 
https://github.com/riscv-non-isa/riscv-iommu/releases/download/v1.0/riscv-iommu.pdf
+[2] 
https://lore.kernel.org/linux-riscv/cover.1718388908.git.tjeznach@rivosinc.com/
diff --git a/docs/system/riscv/virt.rst b/docs/system/riscv/virt.rst
index 9a06f95a34..8e9a2e4dda 100644
--- a/docs/system/riscv/virt.rst
+++ b/docs/system/riscv/virt.rst
@@ -84,6 +84,19 @@ none``, as in
 
 Firmware images used for pflash must be exactly 32 MiB in size.
 
+riscv-iommu support
+-------------------
+
+The board has support for the riscv-iommu-pci device by using the following
+command line:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -M virt -device riscv-iommu-pci (...)
+
+Refer to :ref:`riscv-iommu` for more information on how the RISC-V IOMMU 
support
+works.
+
 Machine-specific options
 ------------------------
 
-- 
2.45.2