[PATCH v4 2/3] Documentation: riscv: Add early boot document

[Palmer Dabbelt]

On Mon, 26 Jun 2023 07:36:25 PDT (-0700), alexghiti at rivosinc.com wrote:
> This document describes the constraints and requirements of the early
> boot process in a RISC-V kernel.
>
> Signed-off-by: Alexandre Ghiti <alexghiti at rivosinc.com>
> Reviewed-by: Björn Töpel <bjorn at rivosinc.com>
> Reviewed-by: Conor Dooley <conor.dooley at microchip.com>
> Reviewed-by: Sunil V L <sunilvl at ventanamicro.com>
> Reviewed-by: Andrew Jones <ajones at ventanamicro.com>
> Reviewed-by: Palmer Dabbelt <palmer at rivosinc.com>
> Reviewed-by: Atish Patra <atishp at rivosinc.com>
> Reviewed-by: Song Shuai <songshuaishuai at tinylab.org>
> Acked-by: Palmer Dabbelt <palmer at rivosinc.com>
> ---
>  Documentation/riscv/boot-image-header.rst |   3 -
>  Documentation/riscv/boot.rst              | 169 ++++++++++++++++++++++
>  Documentation/riscv/index.rst             |   1 +
>  3 files changed, 170 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..f890ac442c91
> --- /dev/null
> +++ b/Documentation/riscv/boot.rst
> @@ -0,0 +1,169 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +===============================================
> +RISC-V Kernel Boot Requirements and Constraints
> +===============================================
> +
> +:Author: Alexandre Ghiti <alexghiti at rivosinc.com>
> +:Date: 23 May 2023
> +
> +This document describes what the RISC-V kernel expects from bootloaders and
> +firmware, but 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 entrance
> +---------------
> +
> +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 Documentation/arm/uefi.rst.
> +
> +Virtual mapping installation
> +----------------------------
> +
> +The installation of the virtual mapping is done in 2 steps in the RISC-V kernel:
> +
> +1. ``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).
> +
> +2. ``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.
> 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

Acked-by: Palmer Dabbelt <palmer at rivosinc.com>