RISC-V Kernel Boot Requirements and Constraints¶

Author:: Alexandre Ghiti <alexghiti@rivosinc.com>
Date:: 23 May 2023

This document describes what the RISC-V kernel expects from bootloaders and firmware, and also the constraints that any developer must have in mind when touching the early boot process. For the purposes of this document, the early boot process refers to any code that runs before the final virtual mapping is set up.

Pre-kernel Requirements and Constraints¶

The RISC-V kernel expects the following of bootloaders and platform firmware:

Register state¶

The RISC-V kernel expects:

$a0 to contain the hartid of the current core.

$a1 to contain the address of the devicetree in memory.

CSR state¶

The RISC-V kernel expects:

$satp = 0: the MMU, if present, must be disabled.

Reserved memory for resident firmware¶

The RISC-V kernel must not map any resident memory, or memory protected with PMPs, in the direct mapping, so the firmware must correctly mark those regions as per the devicetree specification and/or the UEFI specification.

Kernel location¶

The RISC-V kernel expects to be placed at a PMD boundary (2MB aligned for rv64 and 4MB aligned for rv32). Note that the EFI stub will physically relocate the kernel if that's not the case.

Hardware description¶

The firmware can pass either a devicetree or ACPI tables to the RISC-V kernel.

The devicetree is either passed directly to the kernel from the previous stage using the $a1 register, or when booting with UEFI, it can be passed using the EFI configuration table.

The ACPI tables are passed to the kernel using the EFI configuration table. In this case, a tiny devicetree is still created by the EFI stub. Please refer to "EFI stub and devicetree" section below for details about this devicetree.

Kernel entry¶

On SMP systems, there are 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 mostly used to support older firmwares without SBI HSM extension and M-mode RISC-V kernel.
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. The ordered booting method is the preferred booting method for booting the RISC-V kernel because it can support CPU hotplug and kexec.

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 devicetree node and abide by the devicetree 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 devicetree specification v0.4-rc1).

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 devicetree subnode (deprecated).

Any new firmware must implement RISCV_EFI_BOOT_PROTOCOL as the devicetree based approach is deprecated now.

Early Boot Requirements and Constraints¶

The RISC-V kernel's early boot process operates under the following constraints:

EFI stub and devicetree¶

When booting with UEFI, the devicetree is supplemented (or created) by the EFI stub with the same parameters as arm64 which are described at the paragraph "UEFI kernel support on ARM" in The Unified Extensible Firmware Interface (UEFI).

Virtual mapping installation¶

The installation of the virtual mapping is done in 2 steps in the RISC-V kernel:

setup_vm() installs a temporary kernel mapping in early_pg_dir which allows discovery of 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 early_pg_dir page table is statically allocated (using only one table for each level).
setup_vm_final() creates the final kernel mapping in 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 virt_to_phys() and phys_to_virt() to be able to correctly convert direct mapping addresses to physical addresses, they need to know the start of the DRAM. This happens after step 1, right before step 2 installs the direct mapping (see setup_bootmem() function in arch/riscv/mm/init.c). Any usage of those macros before the final virtual mapping is installed must be carefully examined.

Devicetree mapping via fixmap¶

As the reserved_mem array is initialized with virtual addresses established by setup_vm(), and used with the mapping established by setup_vm_final(), the RISC-V kernel uses the fixmap region to map the devicetree. This ensures that the devicetree remains accessible by both virtual mappings.

Pre-MMU execution¶

A few pieces of code need to run before even the first virtual mapping is established. These are the installation of the first virtual mapping itself, patching of early alternatives and the early parsing of the kernel command line. That code 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.
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.