Signed-off-by: Gregor Haas <gregorhaas1997@gmail.com>
---
 MAINTAINERS                           |   1 +
 docs/system/riscv/opensbi_domains.rst | 156 ++++++++++++++++++++++++++
 docs/system/target-riscv.rst          |  10 ++
 3 files changed, 167 insertions(+)
 create mode 100644 docs/system/riscv/opensbi_domains.rst

diff --git a/MAINTAINERS b/MAINTAINERS
index 3b9f5b7432..5eb65cc499 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -361,6 +361,7 @@ RISC-V OpenSBI domain support
 M: Gregor Haas <gregorhaas1997@gmail.com>
 L: qemu-riscv@nongnu.org
 S: Maintained
+F: docs/system/riscv/opensbi_domains.rst
 F: hw/riscv/opensbi_domain.c
 F: include/hw/riscv/opensbi_domain.h
 
diff --git a/docs/system/riscv/opensbi_domains.rst b/docs/system/riscv/opensbi_domains.rst
new file mode 100644
index 0000000000..b8bbf52738
--- /dev/null
+++ b/docs/system/riscv/opensbi_domains.rst
@@ -0,0 +1,156 @@
+OpenSBI Domains
+===============
+
+OpenSBI has support for domains, which are partitions of CPUs and memory into
+isolated compartments. Domains can be specified in the device tree according to
+a standardized format [1_], which OpenSBI parses at boot time to initialize all
+system domains. Depending on the specific QEMU machine being used, these domain
+configurations can be specified on the QEMU command line. Currently, this is
+only possible for the ``virt`` machine. To enable this functionality for a new
+machine, the initialization code must call ``create_fdt_opensbi_domains`` after
+all device tree nodes for peripherals have been initialized. This is to ensure
+that references to devices work for MMIO regions.
+
+There are two "devices" that are used to configure OpenSBI domains with this
+mechanism: ``opensbi-memregion`` and ``opensbi-domain``.
+
+Memregions
+----------
+
+OpenSBI memregions can be added to the machine's device tree with the following
+flag:
+
+.. code-block:: bash
+
+    -device opensbi-memregion
+
+For this device flag, the following options are implemented:
+
+- ``id``: The name of the memregion. This name is later used to link this
+  region to a domain if desired, in which case this argument is required.
+- ``base`` (required): The base address of this memregion. This address must
+  be aligned to ``2 ^ order``.
+- ``order`` (required): The ``log2`` of the memregion's size, which must be
+  between 3 and ``__riscv_xlen`` inclusive.
+- ``mmio`` (optional): A boolean indicating whether the specified physical
+  address range belongs to an MMIO-mapped peripheral device.
+- ``deviceX`` (optional): If ``mmio`` is indicated, this is the device tree
+  path to the ``X``-th device corresponding to this physical address range,
+  where ``0 <= X < OPENSBI_MEMREGION_DEVICES_MAX`` (default 16).
+
+Domains
+-------
+
+OpenSBI domains can be added to the machine's device tree with the following
+flag:
+
+.. code-block:: bash
+
+    - device opensbi-domain
+
+For this device flag, the following options are implemented:
+
+- ``id`` (required): The name of the domain, which becomes its identifier in
+  the device tree
+- ``boot-hart`` (optional): The HART booting the domain instance.
+- ``possible-harts`` (optional): The contiguous list of CPUs for the domain
+  instance, specified as ``firstcpu[-lastcpu]`` (e.g. ``0-3``).
+- ``next-arg1`` (optional): The 64 bit next booting stage arg1 for the domain
+  instance.
+- ``next-addr`` (optional): The 64 bit next booting stage address for the
+  domain instance.
+- ``next-mode`` (optional): The 32 bit next booting stage mode for the domain
+  instance.
+- ``system-reset-allowed`` (optional): Whether the domain instance is allowed
+  to do system reset.
+- ``system-suspend-allowed`` (optional): Whether the domain instance is allowed
+  to do system suspend.
+
+Furthermore, memregions can be linked to domains using the following options:
+
+- ``regionX`` (optional): The ``id`` of the ``X``-th region for this domain,
+  where ``0 <= X < OPENSBI_DOMAIN_MEMREGIONS_MAX`` (default 16).
+- ``permsX`` (optional): Access permissions for the ``X``-th region for this
+  domain, ``0 <= X < OPENSBI_DOMAIN_MEMREGIONS_MAX`` (default 16). This must be
+  encoded using OpenSBI's permission encoding scheme in ``sbi_domain.h``, and
+  copied below at the time of writing for convenience
+
+.. code-block:: c
+
+    /** Flags representing memory region attributes */
+    #define SBI_MEMREGION_M_READABLE	(1UL << 0)
+    #define SBI_MEMREGION_M_WRITABLE	(1UL << 1)
+    #define SBI_MEMREGION_M_EXECUTABLE	(1UL << 2)
+    #define SBI_MEMREGION_SU_READABLE	(1UL << 3)
+    #define SBI_MEMREGION_SU_WRITABLE	(1UL << 4)
+    #define SBI_MEMREGION_SU_EXECUTABLE	(1UL << 5)
+
+Example
+-------
+
+A complete example command line is shown below:
+
+.. code-block:: bash
+
+    $ qemu-system-riscv64 -machine virt -bios fw_jump.bin -cpu max -smp 2 -m 4G -nographic \
+            -device opensbi-memregion,id=mem,base=0xBC000000,order=26,mmio=false \
+            -device opensbi-memregion,id=uart,base=0x10000000,order=12,mmio=true,device0="/soc/serial@10000000" \
+            -device opensbi-domain,id=domain,possible-harts=0-1,boot-hart=0x0,next-addr=0xBC000000,next-mode=1,region0=mem,perms0=0x3f,region1=uart,perms1=0x3f
+
+As a result of the above configuration, QEMU will add the following subnodes to
+the device tree:
+
+.. code-block:: dts
+
+    chosen {
+            opensbi-domains {
+                    compatible = "opensbi,domain,config";
+
+                    domain {
+                            next-mode = <0x01>;
+                            next-addr = <0x00 0xbc000000>;
+                            boot-hart = <0x03>;
+                            regions = <0x8000 0x3f 0x8002 0x3f>;
+                            possible-harts = <0x03 0x01>;
+                            phandle = <0x8003>;
+                            compatible = "opensbi,domain,instance";
+                    };
+
+                    uart {
+                            phandle = <0x8002>;
+                            devices = <0x1800000>;
+                            mmio;
+                            order = <0x0c>;
+                            base = <0x00 0x10000000>;
+                            compatible = "opensbi,domain,memregion";
+                    };
+
+                    mem {
+                            phandle = <0x8000>;
+                            order = <0x1a>;
+                            base = <0x00 0xbc000000>;
+                            compatible = "opensbi,domain,memregion";
+                    };
+            };
+    };
+
+This results in OpenSBI output as below, where regions 01-03 are inherited from
+the root domain and regions 00 and 04 correspond to the user specified ones:
+
+.. code-block:: console
+
+    Domain1 Name              : domain
+    Domain1 Boot HART         : 0
+    Domain1 HARTs             : 0,1
+    Domain1 Region00          : 0x0000000010000000-0x0000000010000fff M: (I,R,W,X) S/U: (R,W,X)
+    Domain1 Region01          : 0x0000000002000000-0x000000000200ffff M: (I,R,W) S/U: ()
+    Domain1 Region02          : 0x0000000080080000-0x000000008009ffff M: (R,W) S/U: ()
+    Domain1 Region03          : 0x0000000080000000-0x000000008007ffff M: (R,X) S/U: ()
+    Domain1 Region04          : 0x00000000bc000000-0x00000000bfffffff M: (R,W,X) S/U: (R,W,X)
+    Domain1 Next Address      : 0x00000000bc000000
+    Domain1 Next Arg1         : 0x0000000000000000
+    Domain1 Next Mode         : S-mode
+    Domain1 SysReset          : no
+    Domain1 SysSuspend        : no
+
+.. _1: https://github.com/riscv-software-src/opensbi/blob/master/docs/domain_support.md
diff --git a/docs/system/target-riscv.rst b/docs/system/target-riscv.rst
index ba195f1518..bb776ea5f3 100644
--- a/docs/system/target-riscv.rst
+++ b/docs/system/target-riscv.rst
@@ -92,3 +92,13 @@ the images they need.
 * ``-bios <file>``
 
 Tells QEMU to load the specified file as the firmware.
+
+RISC-V Devices
+--------------
+
+There are some RISC-V specific devices that can be specified:
+
+.. toctree::
+    :maxdepth: 1
+
+    riscv/opensbi_domains
-- 
2.46.0