[RFC PATCH 0/1] Add boot hartid to a Device tree

Thu Mar 5 04:22:33 CET 2020

Hi,

I have started to implement the corresponding changes in EDK2: https://github.com/changab/edk2-staging-riscv/compare/5f63e9249751ccb9302514455b9a1a7038f34547...RISC-V-DT-fixup
What happens is: The DTB is embedded in the FW image and passed to sbi_init in SEC phase. We initialize OpenSBI as early as possible and because it also wants to modify the device tree, I have to pass it the DTB as next_arg1.
Then it's passed to PEI in the firmware context and further to DXE via a HOB. In DXE the boot-hartid is inserted and it's installed into the EFI system config table from where the EFISTUB reads it.

Unfortunately I don't get any console output after the EFISTUB calls ExitBootServices, so my patch is still WIP.
Atish, which platform file in OpenSBI did you use to test your changes? platform/qemu/virt/platform.c or platform/sifive/fu540/platform.c ?
Maybe the failure is unrelated to the new patches - we've never booted Linux from EDKII yet.

I'm a bit skeptical whether DT is the best way to pass the boothartid to the kernel. Ard has argued that it's good because it's independent from UEFI, but the proper kernel doesn't read the hartid from there - it gets it from a0. Only the EFISTUB reads the hartid from the device tree. Therefore the solution we need is EFI specific anyways, right?
One of the design goals is that we don't want to force bootloaders, which load the EFISTUB (e.g. UEFI Shell, grub chainloading, systemd-boot), to change.

If we let the firmware embed the hartid in the DT, the user cannot swap out the DT later for their custom one. They need to use the one given by the firmware.
Of course we could add commands and config to bootloaders to load and fixup (embed hartid) the device tree... but, as mentioned earlier, we want to avoid that.
Additionally from EDKII side we're also planning to run later stages, including the bootloader, in S-Mode, where they wouldn't even have access to mhartid and thus couldn't fixup the DT.

If instead the firmware writes the hartid into the EFI system config table, the EFISTUB can read it from there, just like it does the device tree already. Then bootloaders can put a user supplied DT in the config table, too, like they always have.

What do you all think - does that make sense?

- Daniel
On 2/25/20 10:07 AM, Ard Biesheuvel wrote:

On Tue, 25 Feb 2020 at 09:59, Chang, Abner (HPS SW/FW Technologist)
<abner.chang at hpe.com><mailto:abner.chang at hpe.com> wrote:

-----Original Message-----
From: Ard Biesheuvel [mailto:ard.biesheuvel at linaro.org]
Sent: Tuesday, February 25, 2020 4:48 PM
To: Chang, Abner (HPS SW/FW Technologist) <abner.chang at hpe.com><mailto:abner.chang at hpe.com>
Cc: Atish Patra <atishp at atishpatra.org><mailto:atishp at atishpatra.org>; Heinrich Schuchardt
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List <u-boot at lists.denx.de><mailto:u-boot at lists.denx.de>; Alexander Graf <agraf at csgraf.de><mailto:agraf at csgraf.de>; Anup Patel
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Hershberger <joe.hershberger at ni.com><mailto:joe.hershberger at ni.com>; Loic Pallardy
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<daniel.schaefer at hpe.com><mailto:daniel.schaefer at hpe.com>
Subject: Re: [RFC PATCH 0/1] Add boot hartid to a Device tree

On Tue, 25 Feb 2020 at 09:28, Chang, Abner (HPS SW/FW Technologist)
<abner.chang at hpe.com><mailto:abner.chang at hpe.com> wrote:

-----Original Message-----
From: Atish Patra [mailto:atishp at atishpatra.org]

<snip header soup>

On Mon, Feb 24, 2020 at 3:35 PM Ard Biesheuvel
<ard.biesheuvel at linaro.org><mailto:ard.biesheuvel at linaro.org> wrote:

On Tue, 25 Feb 2020 at 00:22, Heinrich Schuchardt
<xypron.glpk at gmx.de><mailto:xypron.glpk at gmx.de>

wrote:

On 2/24/20 11:19 PM, Atish Patra wrote:

The RISC-V booting protocol requires the hart id to be present in

"a0"

register. This is not a problem for bootm/booti commands as
they directly jump to Linux kernel. However, bootefi jumps to
a EFI boot stub code in Linux kernel which acts a loader and
jumps to real Linux after terminating the boot services. This
boot stub code has to be aware of the boot hart id so that it
can set it in "a0" before jumping to Linux kernel. Currently,
UEFI protocol doesn't have any mechanism to pass the boot hart
id to an EFI executable. We should keep it this way as this is
a RISC-V specific requirement rather than a UEFI requirement.
Out of the all

possible options, device tree seemed to be the best choice to do this job.

The detailed discussion can be found in the following thread.

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The above mentioned patch is obsoleted by the new suggestion.

Thanks for pointing that out to avoid confusion.

This patch updates the device tree in arch_fixup_fdt() which
is common for all booting commands. As a result, the DT
modification doesn't require any efi related arch specific
functions and all DT related modifications are contained at
one place. However, the hart id node will be available for
Linux even if the kernel is booted using

bootm command.

If that is not acceptable, we can always move the code to an
efi specific function.

Does a related Linux patch already exist?

Yes. But in my local tree ;). It will be included in RISC-V EFI stub
support series which I am planning to post in a couple of days.

How about EDK2?

RISC-V is not supported at all yet in EDK2.

The EDK2 patches are out there and reviewed. I guess it will be
available in mainline EDK2 pretty soon.

Yes, currently we are working on edk2 CI testing for RISCV64 arch.  We

hope edk2 RISC-V port could be in mainstream in Mar.

Excellent! Is this core support? Or do you have a platform implemented as
well that can be upstreamed?

Yes we do have platform implementations to be upstreamed, below is the latest status of RISC-V edk2 port. We will have to update status later because we just merged OpenSBI tag 0.6 to edk2 RISC-V.
https://github.com/riscv/riscv-uefi-edk2-docs

Good to know! I saw some patches going by on the mailing list, but it
is hard to derive the current state of affairs from that.

I'm glad to see you did not make the same mistake we made on ARM and
omit the PEI phase entirely.

What I did notice is the use of APRIORI PEI and APRIORI DXE sections
in your platform descriptions. I recommend you try to avoid that, as
it is a maintenance burden going forward: instead, please use dummy
protocols and NULL library class resolutions if you need to make
generic components depend on platform specific protocols. Also, please
document this - the APRIORI section does not explain *why* you have to
circumvent the ordinary dependency tree based module dispatch.