Re: [PATCH 1/2] Documentation: riscv: Add early boot document

[Alexandre Ghiti]

On Tue, Jun 20, 2023 at 12:32 PM Song Shuai <songshuaishuai@xxxxxxxxxxx> wrote:
>
>
>
> 在 2023/6/19 17:47, Alexandre Ghiti 写道:
> > This document describes the constraints and requirements of the early
> > boot process in a RISC-V kernel.
> >
> > Szigned-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > ---
> >   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@xxxxxxxxxxxx>
> > +: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).
> how about declare the version of Device Tree specification?
> like the devicetree-specification-v0.4-rc1 we recently reference

You're right, the section number refers to this version of the
specification, so it should be noted.

> > +
> > +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
> how about use "string" to declare the type of "bootargs"
> and replace the "Size" in header with "Type"

Why not, this will be more accurate.

> > +==========================  ======   ===========================================
> > +
> > +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
>
> --
> Thanks
> Song Shuai
>

Thanks Song!