docs: Add initial documentation for domain support

We add initial documentation for OpenSBI domain support to help
RISC-V platform vendors achieve system-level partitioning.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>

2 files changed, 107 insertions, 0 deletions

diff --git a/docs/domain_support.md b/docs/domain_support.md
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+OpenSBI Domain Support
+======================
+
+An OpenSBI domain is a system-level partition (subset) of underlying hardware
+having it's own memory regions (RAM and MMIO devices) and HARTs. The OpenSBI
+will try to achieve secure isolation between domains using RISC-V platform
+features such as PMP, ePMP, IOPMP, SiFive Shield, etc.
+
+Important entities which help implement OpenSBI domain support are:
+
+* **struct sbi_domain_memregion** - Representation of a domain memory region
+* **struct sbi_hartmask** - Representation of domain HART set
+* **struct sbi_domain** - Representation of a domain instance
+
+Each HART of a RISC-V platform must have an OpenSBI domain assigned to it.
+The OpenSBI platform support is responsible for populating domains and
+providing HART id to domain mapping. The OpenSBI domain support will by
+default assign **the ROOT domain** to all HARTs of a RISC-V platform so
+it is not mandatory for the OpenSBI platform support to populate domains.
+
+Domain Memory Region
+--------------------
+
+A domain memory region is represented by **struct sbi_domain_memregion** in
+OpenSBI and has following details:
+
+* **order** - The size of a memory region is **2 ^ order** where **order**
+  must be **3 <= order <= __riscv_xlen**
+* **base** - The base address of a memory region is **2 ^ order**
+  aligned start address
+* **flags** - The flags of a memory region represent memory type (i.e.
+  RAM or MMIO) and allowed accesses (i.e. READ, WRITE, EXECUTE, etc)
+
+Domain Instance
+---------------
+
+A domain instance is represented by **struct sbi_domain** in OpenSBI and
+has following details:
+
+* **index** - Logical index of this domain
+* **name** - Name of this domain
+* **assigned_harts** - HARTs assigned to this domain
+* **possible_harts** - HARTs possible in this domain
+* **regions** - Array of memory regions terminated by a memory region
+  with order zero
+* **boot_hartid** - HART id of the HART booting this domain. The domain
+  boot HART will be started at boot-time if boot HART is possible and
+  assigned for this domain.
+* **next_addr** - Address of the next booting stage for this domain
+* **next_arg1** - Arg1 (or 'a1' register) of the next booting stage for
+  this domain
+* **next_mode** - Privilege mode of the next booting stage for this
+  domain. This can be either S-mode or U-mode.
+* **system_reset_allowed** - Is domain allowed to reset the system?
+
+The memory regions represented by **regions** in **struct sbi_domain** have
+following additional constraints to align with RISC-V PMP requirements:
+
+* A memory region to protect OpenSBI firmware from S-mode and U-mode
+  should always be present
+* For two overlapping memory regions, one should be sub-region of another
+* Two overlapping memory regions should not be of same size
+* Two overlapping memory regions cannot have same flags
+* Memory access checks on overlapping address should prefer smallest
+  overlapping memory region flags.
+
+ROOT Domain
+-----------
+
+**The ROOT domain** is the default OpenSBI domain which is assigned by
+default to all HARTs of a RISC-V platform. The OpenSBI domain support
+will hand-craft **the ROOT domain** very early at boot-time in the
+following manner:
+
+* **index** - Logical index of the ROOT domain is always zero
+* **name** - Name of the ROOT domain is "root"
+* **assigned_harts** - At boot-time all valid HARTs of a RISC-V platform
+  are assigned the ROOT domain which changes later based on OpenSBI
+  platform support
+* **possible_harts** - All valid HARTs of a RISC-V platform are possible
+  HARTs of the ROOT domain
+* **regions** - Two memory regions available to the ROOT domain:
+  **A)** A memory region to protect OpenSBI firmware from S-mode and U-mode
+  **B)** A memory region of **order=__riscv_xlen** allowing S-mode and
+  U-mode access to full memory address space
+* **boot_hartid** - Coldboot HART is the HART booting the ROOT domain
+* **next_addr** - Next booting stage address in coldboot HART scratch
+  space is the next address for the ROOT domain
+* **next_arg1** - Next booting stage arg1 in coldboot HART scratch space
+  is the next arg1 for the ROOT domain
+* **next_mode** - Next booting stage mode in coldboot HART scratch space
+  is the next mode for the ROOT domain
+* **system_reset_allowed** - The ROOT domain is allowed to reset the system
+
+Domain Effects
+--------------
+
+Few noteworthy effects of a system partitioned into domains are as follows:
+
+* At any point in time, a HART is running in exactly one OpenSBI domain context
+* The SBI IPI and RFENCE calls from HART A are restricted to the HARTs in
+  domain assigned to HART A
+* The SBI HSM calls which try to change/read state of HART B from HART A will
+  only work if both HART A and HART B are assigned same domain
+* A HART running in S-mode or U-mode can only access memory based on the
+  memory regions of the domain assigned to the HART
diff --git a/docs/doxygen.cfg b/docs/doxygen.cfg
index dbf8ef9..82f31a7 100644
--- a/docs/doxygen.cfg
+++ b/docs/doxygen.cfg
@@ -795,6 +795,7 @@ INPUT                  = @@SRC_DIR@@/README.md \
                          @@SRC_DIR@@/docs/platform_guide.md \
                          @@SRC_DIR@@/docs/platform_requirements.md \
                          @@SRC_DIR@@/docs/library_usage.md \
+                         @@SRC_DIR@@/docs/domain_support.md \
                          @@SRC_DIR@@/docs/firmware \
                          @@SRC_DIR@@/docs/platform \
                          @@SRC_DIR@@/include \