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From:   Alexandre Ghiti <alexghiti@rivosinc.com>
To:     Jonathan Corbet <corbet@lwn.net>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>, linux-doc@vger.kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     Alexandre Ghiti <alexghiti@rivosinc.com>
Subject: [PATCH 1/2] Documentation: riscv: Add early boot document
Date:   Mon, 19 Jun 2023 11:47:04 +0200
Message-Id: <20230619094705.51337-1-alexghiti@rivosinc.com>
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This document describes the constraints and requirements of the early
boot process in a RISC-V kernel.

Szigned-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 Documentation/riscv/boot-image-header.rst |   3 -
 Documentation/riscv/boot.rst              | 181 ++++++++++++++++++++++
 Documentation/riscv/index.rst             |   1 +
 3 files changed, 182 insertions(+), 3 deletions(-)
 create mode 100644 Documentation/riscv/boot.rst

diff --git a/Documentation/riscv/boot-image-header.rst b/Documentation/riscv/boot-image-header.rst
index d7752533865f..a4a45310c4c4 100644
--- a/Documentation/riscv/boot-image-header.rst
+++ b/Documentation/riscv/boot-image-header.rst
@@ -7,9 +7,6 @@ Boot image header in RISC-V Linux
 
 This document only describes the boot image header details for RISC-V Linux.
 
-TODO:
-  Write a complete booting guide.
-
 The following 64-byte header is present in decompressed Linux kernel image::
 
 	u32 code0;		  /* Executable code */
diff --git a/Documentation/riscv/boot.rst b/Documentation/riscv/boot.rst
new file mode 100644
index 000000000000..b02230818b79
--- /dev/null
+++ b/Documentation/riscv/boot.rst
@@ -0,0 +1,181 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============================================
+Early boot requirements/constraints on RISC-V
+=============================================
+
+:Author: Alexandre Ghiti <alexghiti@rivosinc.com>
+:Date: 23 May 2023
+
+This document describes what the RISC-V kernel expects from the previous stages
+and the firmware, but also the constraints that any developer must have in mind
+when touching the early boot process, e.g. before the final virtual mapping is
+setup.
+
+Pre-kernel boot (Expectations from firmware)
+============================================
+
+Registers state
+---------------
+
+The RISC-V kernel expects:
+
+  * `$a0` to contain the hartid of the current core.
+  * `$a1` to contain the address of the device tree in memory.
+
+CSR state
+---------
+
+The RISC-V kernel expects:
+
+  * `$satp = 0`: the MMU must be disabled.
+
+Reserved memory for resident firmware
+-------------------------------------
+
+The RISC-V kernel expects the firmware to mark any resident memory with the
+`no-map` flag, thus the kernel won't map those regions in the direct mapping
+(avoiding issues with hibernation, speculative accesses and probably other
+subsystems).
+
+Kernel location
+---------------
+
+The RISC-V kernel expects to be placed at a PMD boundary (2MB for rv64 and 4MB
+for rv32). Note though that the EFI stub will physically relocate the kernel if
+that's not the case.
+
+Device-tree
+-----------
+
+The RISC-V kernel always expects a device tree, it is:
+
+- either passed directly to the kernel from the previous stage using the `$a1`
+  register,
+- or when booting with UEFI, the device tree will be retrieved by the EFI stub
+  using the EFI configuration table or it will be created.
+
+Bootflow
+--------
+
+There exist 2 methods to enter the kernel:
+
+- `RISCV_BOOT_SPINWAIT`: the firmware releases all harts in the kernel, one hart
+  wins a lottery and executes the early boot code while the other harts are
+  parked waiting for the initialization to finish. This method is now
+  **deprecated**.
+- Ordered booting: the firmware releases only one hart that will execute the
+  initialization phase and then will start all other harts using the SBI HSM
+  extension.
+
+UEFI
+----
+
+UEFI memory map
+~~~~~~~~~~~~~~~
+
+When booting with UEFI, the RISC-V kernel will use only the EFI memory map to
+populate the system memory.
+
+The UEFI firmware must parse the subnodes of the `/reserved-memory` device tree
+node and abide by the device tree specification to convert the attributes of
+those subnodes (`no-map` and `reusable`) into their correct EFI equivalent
+(refer to section "3.5.4 /reserved-memory and UEFI" of the device tree
+specification).
+
+RISCV_EFI_BOOT_PROTOCOL
+~~~~~~~~~~~~~~~~~~~~~~~
+
+When booting with UEFI, the EFI stub requires the boot hartid in order to pass
+it to the RISC-V kernel in `$a1`. The EFI stub retrieves the boot hartid using
+one of the following methods:
+
+- `RISCV_EFI_BOOT_PROTOCOL` (**preferred**).
+- `boot-hartid` device tree subnode (**deprecated**).
+
+Any new firmware must implement `RISCV_EFI_BOOT_PROTOCOL` as the device tree
+based approach is deprecated now.
+
+During kernel boot: (Kernel internals)
+======================================
+
+EFI stub and device tree
+------------------------
+
+When booting with UEFI, the device tree is supplemented by the EFI stub with the
+following parameters (largely shared with arm64 in Documentation/arm/uefi.rst):
+
+==========================  ======   ===========================================
+Name                        Size     Description
+==========================  ======   ===========================================
+linux,uefi-system-table     64-bit   Physical address of the UEFI System Table.
+
+linux,uefi-mmap-start       64-bit   Physical address of the UEFI memory map,
+                                     populated by the UEFI GetMemoryMap() call.
+
+linux,uefi-mmap-size        32-bit   Size in bytes of the UEFI memory map
+                                     pointed to in previous entry.
+
+linux,uefi-mmap-desc-size   32-bit   Size in bytes of each entry in the UEFI
+                                     memory map.
+
+linux,uefi-mmap-desc-ver    32-bit   Version of the mmap descriptor format.
+
+kaslr-seed                  64-bit   Entropy used to randomize the kernel image
+                                     base address location.
+
+bootargs                             Kernel command line
+==========================  ======   ===========================================
+
+Virtual mapping setup
+---------------------
+
+The installation of the virtual mapping is done in 2 steps in the RISC-V kernel:
+
+1. :c:func:`setup_vm` installs a temporary kernel mapping in
+   :c:var:`early_pg_dir` which allows to discover the system memory: only the
+   kernel text/data are mapped at this point. When establishing this mapping,
+   no allocation can be done (since the system memory is not known yet), so
+   :c:var:`early_pg_dir` page table is statically allocated (using only one
+   table for each level).
+
+2. :c:func:`setup_vm_final` creates the final kernel mapping in
+   :c:var:`swapper_pg_dir` and takes advantage of the discovered system memory
+   to create the linear mapping. When establishing this mapping, the kernel
+   can allocate memory but cannot access it directly (since the direct mapping
+   is not present yet), so it uses temporary mappings in the fixmap region to
+   be able to access the newly allocated page table levels.
+
+For :c:func:`virt_to_phys` and :c:func:`phys_to_virt` to be able to correctly
+convert direct mapping addresses to physical addresses, it needs to know the
+start of the DRAM: this happens after 1, right before 2 installs the direct
+mapping (see :c:func:`setup_bootmem` function in arch/riscv/mm/init.c). So
+any usage of those macros before the final virtual mapping is installed must be
+carefully examined.
+
+Device-tree mapping via fixmap
+------------------------------
+
+The RISC-V kernel uses the fixmap region to map the device tree because the
+device tree virtual mapping must remain the same between :c:func:`setup_vm` and
+:c:func:`setup_vm_final` calls since :c:var:`reserved_mem` array is initialized
+with virtual addresses established by :c:func:`setup_vm` and used with the
+mapping established by :c:func:`setup_vm_final`.
+
+Pre-MMU execution
+-----------------
+
+Any code that executes before even the first virtual mapping is established
+must be very carefully compiled as:
+
+- `-fno-pie`: This is needed for relocatable kernels which use `-fPIE`, since
+  otherwise, any access to a global symbol would go through the GOT which is
+  only relocated virtually.
+- `-mcmodel=medany`: Any access to a global symbol must be PC-relative to avoid
+  any relocations to happen before the MMU is setup.
+- Also note that *all* instrumentation must also be disabled (that includes
+  KASAN, ftrace and others).
+
+As using a symbol from a different compilation unit requires this unit to be
+compiled with those flags, we advise, as much as possible, not to use external
+symbols.
diff --git a/Documentation/riscv/index.rst b/Documentation/riscv/index.rst
index 175a91db0200..1f66062def6d 100644
--- a/Documentation/riscv/index.rst
+++ b/Documentation/riscv/index.rst
@@ -5,6 +5,7 @@ RISC-V architecture
 .. toctree::
     :maxdepth: 1
 
+    boot
     boot-image-header
     vm-layout
     hwprobe
-- 
2.39.2