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

[Conor Dooley]

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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