Hey Alex, Thanks for working on this :) I've got a mix of suggestions and questions below. Hopefully it is not too disjoint, since I didn't write them in order. On Mon, Jun 19, 2023 at 11:47:04AM +0200, Alexandre Ghiti wrote: > 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 > +============================================= Please use "title case", here and elsewhere in the doc. I'd also be inclined to drop the "Early" from here, as it permits more natural section headings. Perhaps "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 the previous stages "the previous stages" is a bit vague IMO. You mean bootloader stages I assume, but I think it should be explicit. Perhaps: "...what a RISC-V kernel expects from bootloaders and firmware, and the constraints..." > +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. s/setup./set up./ Do you mean to have "For example" here? Or is "before the final virtual mapping is set up" the definition or "early boot"? If the latter, I would reword this as something like: "...when modifying 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 boot (Expectations from firmware) Firmware or bootloaders? TBH, I would just drop the section in () and do something like: Pre-kernel Requirements and Constraints ======================================= The RISC-V kernel expects the following of bootloaders and platform firmware: > + > +Registers state s/Registers state/Register 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: > + > + * `$satp = 0`: the MMU must be disabled. "the MMU, if present, must be disabled." ;) > + > +Reserved memory for resident firmware > +------------------------------------- > + > +The RISC-V kernel expects the firmware to mark any resident memory with the Should this be "...resident memory, or memory it has protected with PMPs, with..." ? > +`no-map` flag, thus the kernel won't map those regions in the direct mapping "no-map" is a DT specific term, should this section be moved down under DT, as a sub-section of that? > +(avoiding issues with hibernation, speculative accesses and probably other > +subsystems). I'm not sure that this () section is beneficial. To be honest, recent issues aside, this section here seems like a statement of the obvious... > + > +Kernel location > +--------------- > + > +The RISC-V kernel expects to be placed at a PMD boundary (2MB for rv64 and 4MB Would that be better worded as "(2 MB aligned for rv64 and 4 MB aligned for rv32)"? It might be overly explicit, but I figure there's no harm... > +for rv32). Note though that the EFI stub will physically relocate the kernel if s/though// > +that's not the case. > + > +Device-tree s/Device-tree/Devicetree/ and... > +----------- > + > +The RISC-V kernel always expects a device tree, it is: ...s/device tree/devicetree/ to match elsewhere in the kernel docs. Same applies to the other instances of "device tree" in this patch, please. > + > +- either passed directly to the kernel from the previous stage using the `$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. Can I suggest changing this around a little, pulling the "either" & dropping some boilerplate so that it reads (to me!) a little more naturally: The RISC-V kernel always expects a devicetree, it is either: - passed directly to the kernel from the previous stage using the `$a1` register, - retrieved by the EFI stub when booting with UEFI, using the EFI configuration table or it will be created by ____. Also, please elaborate on what it will be created by. > + > +Bootflow "Boot Flow", no? I am not sure that this is the "correct" heading for the content it describes, but I have nothing better to offer :/ > +-------- > + > +There exist 2 methods to enter the kernel: > + > +- `RISCV_BOOT_SPINWAIT`: the firmware releases all harts in the kernel, one hart > + wins a lottery and executes the early boot code while the other harts are > + parked waiting for the initialization to finish. This method is now nit: s/now// What do you mean by deprecated? There's no requirement to implement the HSM extension, right? > + **deprecated**. > +- Ordered booting: the firmware releases only one hart that will execute the > + initialization phase and then will start all other harts using the SBI HSM > + extension. > + > +UEFI > +---- > + > +UEFI memory map > +~~~~~~~~~~~~~~~ > + > +When booting with UEFI, the RISC-V kernel will use only the EFI memory map to > +populate the system memory. > + > +The UEFI firmware must parse the subnodes of the `/reserved-memory` device tree > +node and abide by the device tree specification to convert the attributes of > +those subnodes (`no-map` and `reusable`) into their correct EFI equivalent > +(refer to section "3.5.4 /reserved-memory and UEFI" of the device tree > +specification). > + > +RISCV_EFI_BOOT_PROTOCOL > +~~~~~~~~~~~~~~~~~~~~~~~ > + > +When booting with UEFI, the EFI stub requires the boot hartid in order to pass > +it to the RISC-V kernel in `$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) With the other section titles changed, this could be: Early Boot Requirements and Constraints ======================================= The RISC-V kernel's early boot process operates under the following constraints: Thoughts? > +====================================== > + > +EFI stub and device tree Same comments about "device tree" here etc. > +------------------------ > + > +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. nit: Hmm, I think for all of these sizes s/-bit/ bits/. > + > +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 > +========================== ====== =========================================== > + > +Virtual mapping setup nit: s/setup/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 to discover the system memory: only the s/to discover/discovery of/ s/: only/. Only/ > + 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 nit: s/it/they/ > +start of the DRAM: this happens after 1, right before 2 installs the direct s/:/./ Also how about s/1/step 1/ & s/2/step 2/? > +mapping (see :c:func:`setup_bootmem` function in arch/riscv/mm/init.c). So s/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 Missing a "the" before reserved_mem. > +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: Could you point out what the non-obvious examples of this code are? > +- `-fno-pie`: This is needed for relocatable kernels which use `-fPIE`, since Is there a reason why the capitalisation is different for the two compiler flags? > + 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 nit: s/Also note that// > + 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. If the use of early alternatives grows, are we going to have to split the vendors early alternatives into a different compilation unit from their regular alternatives? Cheers, Conor.
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