Re: [PATCH v2 2/3] Documentation: riscv: Add early boot document

[Andrew Jones]

On Fri, Jun 23, 2023 at 10:48:55AM +0200, Alexandre Ghiti wrote:
> 
> On 21/06/2023 14:19, Andrew Jones wrote:
> > On Wed, Jun 21, 2023 at 09:22:33AM +0200, Alexandre Ghiti wrote:
> > > This document describes the constraints and requirements of the early
> > > boot process in a RISC-V kernel.
> > > 
> > > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > > Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
> > > ---
> > >   Documentation/riscv/boot-image-header.rst |   3 -
> > >   Documentation/riscv/boot.rst              | 170 ++++++++++++++++++++++
> > >   Documentation/riscv/index.rst             |   1 +
> > >   3 files changed, 171 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..019ee818686d
> > > --- /dev/null
> > > +++ b/Documentation/riscv/boot.rst
> > > @@ -0,0 +1,170 @@
> > > +.. SPDX-License-Identifier: GPL-2.0
> > > +
> > > +===============================================
> > > +RISC-V Kernel Boot Requirements and Constraints
> > > +===============================================
> > > +
> > > +:Author: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > > +: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" tree section below for details about this devicetree.
> >                               ^ redundant 'tree' here
> > 
> > > +
> > > +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. :c:func:`setup_vm` installs a temporary kernel mapping in
> > > +   :c:var:`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
> > > +   :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, they need to know the
> > > +start of the DRAM. This happens after step 1, right before step 2 installs the
> > > +direct mapping (see :c:func:`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.
> > > +
> > > +Device-tree mapping via fixmap
> > > +------------------------------
> > > +
> > > +The RISC-V kernel uses the fixmap region to map the devicetree because the
> > > +devicetree virtual mapping must remain the same between :c:func:`setup_vm` and
> > > +:c:func:`setup_vm_final` calls since the :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
> > > +-----------------
> > > +
> > > +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
> > > -- 
> > > 2.39.2
> > > 
> > Otherwise looks good to me.
> 
> 
> I assume I can add your RB here, don't waste time answering if I'm right :)

Yup, when I read it the first time I wasn't sure I was familiar enough
with everything to give it an r-b, but I just read it again and grepped
a few things that I wasn't already familiar with, so

Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>

Thanks,
drew