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!
