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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | =============================================== | |
4 | RISC-V Kernel Boot Requirements and Constraints | |
5 | =============================================== | |
6 | ||
7 | :Author: Alexandre Ghiti <alexghiti@rivosinc.com> | |
8 | :Date: 23 May 2023 | |
9 | ||
10 | This document describes what the RISC-V kernel expects from bootloaders and | |
11 | firmware, and also the constraints that any developer must have in mind when | |
12 | touching the early boot process. For the purposes of this document, the | |
13 | ``early boot process`` refers to any code that runs before the final virtual | |
14 | mapping is set up. | |
15 | ||
16 | Pre-kernel Requirements and Constraints | |
17 | ======================================= | |
18 | ||
19 | The RISC-V kernel expects the following of bootloaders and platform firmware: | |
20 | ||
21 | Register state | |
22 | -------------- | |
23 | ||
24 | The RISC-V kernel expects: | |
25 | ||
26 | * ``$a0`` to contain the hartid of the current core. | |
27 | * ``$a1`` to contain the address of the devicetree in memory. | |
28 | ||
29 | CSR state | |
30 | --------- | |
31 | ||
32 | The RISC-V kernel expects: | |
33 | ||
34 | * ``$satp = 0``: the MMU, if present, must be disabled. | |
35 | ||
36 | Reserved memory for resident firmware | |
37 | ------------------------------------- | |
38 | ||
39 | The RISC-V kernel must not map any resident memory, or memory protected with | |
40 | PMPs, in the direct mapping, so the firmware must correctly mark those regions | |
41 | as per the devicetree specification and/or the UEFI specification. | |
42 | ||
43 | Kernel location | |
44 | --------------- | |
45 | ||
46 | The RISC-V kernel expects to be placed at a PMD boundary (2MB aligned for rv64 | |
47 | and 4MB aligned for rv32). Note that the EFI stub will physically relocate the | |
48 | kernel if that's not the case. | |
49 | ||
50 | Hardware description | |
51 | -------------------- | |
52 | ||
53 | The firmware can pass either a devicetree or ACPI tables to the RISC-V kernel. | |
54 | ||
55 | The devicetree is either passed directly to the kernel from the previous stage | |
56 | using the ``$a1`` register, or when booting with UEFI, it can be passed using the | |
57 | EFI configuration table. | |
58 | ||
59 | The ACPI tables are passed to the kernel using the EFI configuration table. In | |
60 | this case, a tiny devicetree is still created by the EFI stub. Please refer to | |
61 | "EFI stub and devicetree" section below for details about this devicetree. | |
62 | ||
63 | Kernel entry | |
64 | ------------ | |
65 | ||
66 | On SMP systems, there are 2 methods to enter the kernel: | |
67 | ||
68 | - ``RISCV_BOOT_SPINWAIT``: the firmware releases all harts in the kernel, one hart | |
69 | wins a lottery and executes the early boot code while the other harts are | |
70 | parked waiting for the initialization to finish. This method is mostly used to | |
71 | support older firmwares without SBI HSM extension and M-mode RISC-V kernel. | |
72 | - ``Ordered booting``: the firmware releases only one hart that will execute the | |
73 | initialization phase and then will start all other harts using the SBI HSM | |
74 | extension. The ordered booting method is the preferred booting method for | |
75 | booting the RISC-V kernel because it can support CPU hotplug and kexec. | |
76 | ||
77 | UEFI | |
78 | ---- | |
79 | ||
80 | UEFI memory map | |
81 | ~~~~~~~~~~~~~~~ | |
82 | ||
83 | When booting with UEFI, the RISC-V kernel will use only the EFI memory map to | |
84 | populate the system memory. | |
85 | ||
86 | The UEFI firmware must parse the subnodes of the ``/reserved-memory`` devicetree | |
87 | node and abide by the devicetree specification to convert the attributes of | |
88 | those subnodes (``no-map`` and ``reusable``) into their correct EFI equivalent | |
89 | (refer to section "3.5.4 /reserved-memory and UEFI" of the devicetree | |
90 | specification v0.4-rc1). | |
91 | ||
92 | RISCV_EFI_BOOT_PROTOCOL | |
93 | ~~~~~~~~~~~~~~~~~~~~~~~ | |
94 | ||
95 | When booting with UEFI, the EFI stub requires the boot hartid in order to pass | |
96 | it to the RISC-V kernel in ``$a1``. The EFI stub retrieves the boot hartid using | |
97 | one of the following methods: | |
98 | ||
99 | - ``RISCV_EFI_BOOT_PROTOCOL`` (**preferred**). | |
100 | - ``boot-hartid`` devicetree subnode (**deprecated**). | |
101 | ||
102 | Any new firmware must implement ``RISCV_EFI_BOOT_PROTOCOL`` as the devicetree | |
103 | based approach is deprecated now. | |
104 | ||
105 | Early Boot Requirements and Constraints | |
106 | ======================================= | |
107 | ||
108 | The RISC-V kernel's early boot process operates under the following constraints: | |
109 | ||
110 | EFI stub and devicetree | |
111 | ----------------------- | |
112 | ||
113 | When booting with UEFI, the devicetree is supplemented (or created) by the EFI | |
114 | stub with the same parameters as arm64 which are described at the paragraph | |
115 | "UEFI kernel support on ARM" in Documentation/arch/arm/uefi.rst. | |
116 | ||
117 | Virtual mapping installation | |
118 | ---------------------------- | |
119 | ||
120 | The installation of the virtual mapping is done in 2 steps in the RISC-V kernel: | |
121 | ||
122 | 1. ``setup_vm()`` installs a temporary kernel mapping in ``early_pg_dir`` which | |
123 | allows discovery of the system memory. Only the kernel text/data are mapped | |
124 | at this point. When establishing this mapping, no allocation can be done | |
125 | (since the system memory is not known yet), so ``early_pg_dir`` page table is | |
126 | statically allocated (using only one table for each level). | |
127 | ||
128 | 2. ``setup_vm_final()`` creates the final kernel mapping in ``swapper_pg_dir`` | |
129 | and takes advantage of the discovered system memory to create the linear | |
130 | mapping. When establishing this mapping, the kernel can allocate memory but | |
131 | cannot access it directly (since the direct mapping is not present yet), so | |
132 | it uses temporary mappings in the fixmap region to be able to access the | |
133 | newly allocated page table levels. | |
134 | ||
135 | For ``virt_to_phys()`` and ``phys_to_virt()`` to be able to correctly convert | |
136 | direct mapping addresses to physical addresses, they need to know the start of | |
137 | the DRAM. This happens after step 1, right before step 2 installs the direct | |
138 | mapping (see ``setup_bootmem()`` function in arch/riscv/mm/init.c). Any usage of | |
139 | those macros before the final virtual mapping is installed must be carefully | |
140 | examined. | |
141 | ||
142 | Devicetree mapping via fixmap | |
143 | ----------------------------- | |
144 | ||
145 | As the ``reserved_mem`` array is initialized with virtual addresses established | |
146 | by ``setup_vm()``, and used with the mapping established by | |
147 | ``setup_vm_final()``, the RISC-V kernel uses the fixmap region to map the | |
148 | devicetree. This ensures that the devicetree remains accessible by both virtual | |
149 | mappings. | |
150 | ||
151 | Pre-MMU execution | |
152 | ----------------- | |
153 | ||
154 | A few pieces of code need to run before even the first virtual mapping is | |
155 | established. These are the installation of the first virtual mapping itself, | |
156 | patching of early alternatives and the early parsing of the kernel command line. | |
157 | That code must be very carefully compiled as: | |
158 | ||
159 | - ``-fno-pie``: This is needed for relocatable kernels which use ``-fPIE``, | |
160 | since otherwise, any access to a global symbol would go through the GOT which | |
161 | is only relocated virtually. | |
162 | - ``-mcmodel=medany``: Any access to a global symbol must be PC-relative to | |
163 | avoid any relocations to happen before the MMU is setup. | |
164 | - *all* instrumentation must also be disabled (that includes KASAN, ftrace and | |
165 | others). | |
166 | ||
167 | As using a symbol from a different compilation unit requires this unit to be | |
168 | compiled with those flags, we advise, as much as possible, not to use external | |
169 | symbols. |