On Tue, Feb 11, 2020 at 11:28 PM Ard Biesheuvel ard.biesheuvel@linaro.org wrote:
On Wed, 12 Feb 2020 at 06:49, Chang, Abner (HPS SW/FW Technologist) abner.chang@hpe.com wrote:
-----Original Message----- From: Heinrich Schuchardt [mailto:xypron.glpk@gmx.de] Sent: Wednesday, February 12, 2020 2:26 AM To: Chang, Abner (HPS SW/FW Technologist) abner.chang@hpe.com; Atish Patra atishp@atishpatra.org; Ard Biesheuvel ard.biesheuvel@linaro.org Cc: Alexander Graf agraf@csgraf.de; U-Boot Mailing List <u- boot@lists.denx.de>; Atish Patra atish.patra@wdc.com; leif@nuviainc.com Subject: Re: [PATCH v2 1/1] efi_loader: architecture specific UEFI setup
On 2/7/20 4:13 AM, Chang, Abner (HPS SW/FW Technologist) wrote:
-----Original Message----- From: Atish Patra [mailto:atishp@atishpatra.org] Sent: Friday, February 7, 2020 6:56 AM To: Ard Biesheuvel ard.biesheuvel@linaro.org; Chang, Abner (HPS SW/FW Technologist) abner.chang@hpe.com Cc: Alexander Graf agraf@csgraf.de; Heinrich Schuchardt xypron.glpk@gmx.de; U-Boot Mailing List u-boot@lists.denx.de; Atish Patra atish.patra@wdc.com; leif@nuviainc.com Subject: Re: [PATCH v2 1/1] efi_loader: architecture specific UEFI setup
On Thu, Feb 6, 2020 at 2:07 PM Ard Biesheuvel ard.biesheuvel@linaro.org wrote:
On Thu, 6 Feb 2020 at 21:06, Atish Patra atishp@atishpatra.org wrote: > > On Thu, Feb 6, 2020 at 12:10 PM Alexander Graf agraf@csgraf.de
wrote:
>> >> >> On 06.02.20 19:28, Atish Patra wrote: >>> On Tue, Feb 4, 2020 at 11:43 PM Ard Biesheuvel >>> ard.biesheuvel@linaro.org wrote: >>>> On Wed, 5 Feb 2020 at 05:53, Heinrich Schuchardt
xypron.glpk@gmx.de wrote:
>>>>> RISC-V booting currently is based on a per boot stage lottery >>>>> to determine the active CPU. The Hart State Management (HSM) >>>>> SBI extension replaces this lottery by using a dedicated >>>>> primary
CPU.
>>>>> >>>>> Cf. Hart State Management Extension, Extension ID: 0x48534D >>>>> (HSM) >>>>> https://github.com/riscv/riscv-sbi-doc/blob/master/riscv-sbi.a >>>>> doc >>>>> >>>>> In this scenario the id of the active hart has to be passed >>>>> from boot stage to boot stage. Using a UEFI variable would >>>>> provide
an easy implementation.
>>>>> >>>>> This patch provides a weak function that is called at the end >>>>> of the setup of U-Boot's UEFI sub-system. By overriding this >>>>> function architecture specific UEFI variables or configuration >>>>> tables
can be created.
>>>>> >>>>> Signed-off-by: Heinrich Schuchardt xypron.glpk@gmx.de >>>>> Reviewed-by: Atish Patra atish.patra@wdc.com >>>> OK, so I have a couple of questions: >>>> >>>> - does RISC-V use device tree? if so, why are you not passing >>>> the active hart via a property in the /chosen node? >>> Yes. RISC-V uses device tree. Technically, we can pass the active >>> hart by a DT property but that means we have to modify the DT in >>> OpenSBI (RISC-V specific run time service provider). >>> We have been trying to avoid that if possible so that DT is not >>> bounced around. This also limits U-Boot to have its own device >>> tree. >> >> >> I don't understand how this is different from the UEFI variable >> scheme proposed here? If you want to create an SBI interface to >> propagate the active HART that U-Boot then uses to populate the >> /chosen property, that's probably fine as well. >> > > We don't want to create SBI interface to pass this information. > >> We already have hooks that allow you to modify the DT right before >> it gets delivered to the payload. Just add it there? >> > > Hmm. I guess it is true if the DT is loaded from MMC or network as well. > How about EDK2 ? If we go DT route, it also has to modify the DT to > pass the boot hart. > > As it requires DT modification in multiple projects, why not use > efi configuration tables as suggested by Ard ? >
Configuration tables are preferred over variables, but putting it in the DT makes even more sense, since in that case, nothing that runs in the UEFI context has to care about any of this.
>>> >>> >>>> I'd assume the EFI stub would not care at all about this >>>> information, and it would give you a Linux/RISC-V specific way >>>> to convey this information that is independent of EFI. >>> Yes. EFI stub doesn't care about this information. However, it >>> needs to save the information somewhere so that it can pass to >>> the real kernel after exiting boot time services. >> >> >> DT sounds like a pretty natural choice for that to me :). >>
Indeed. DT has a /chosen node that is set aside for this purpose. It does depend on how early you need the value (i.e., before or after you can run C code), but since you are passing the DT address to the core kernel, it makes way more sense to drop any additional information that you need to pass in there.
We don't need boot hart id until real kernel boots and parse DT. So that should be okay. I just looked at the efi stub code once more and realized that it is already parsing the DT to setup uefi memory maps from /chosen node. Adding boot hart id to the chosen node does seem much cleaner to me :). Thanks for all the explanations.
I have not looked at EDK2 code. But I am assuming modifying the DT just before jumping to the payload won't be too hard for EDK2 as well.
We don’t use DT in edk2 RISC-V port and we pass boot HART ID in SMBIOS type 44h as it is spec out in below link, https://github.com/riscv/riscv-smbios/blob/master/RISCV-SMBIOS.md
Thanks for the link.
For 'RISC-V SMBIOS Type 44 Processor Additional Information' I find entry 0x13h 1: This is boot hart to boot system .
But is '44' a hexadecimal number? The document does not indicate this.
Type '44' is decimal format as it mentioned in SMBIOS spec, I had typo in above which said '44h'. However, that's good to mention this in RISCV_SMBIOS.md. Thanks for the recommendation.
SMBIOS data is intended to describe the hardware to system administrators, not to the OS loader, and I don't think it makes sense to rely on it for booting. I'd assume that SMBIOS tables are not mandatory to begin with.
For EFI boot, it is acceptable if the stub loader in Linux itself needs to obtain the value from something like a device tree and pass it in a CPU register at handover time,
That's what I am planning to do for now. We can add SMBIOS parsing as well if required in future.
although I would still prefer
it if the kernel simply gets it from the device tree directly if one is guaranteed to be available.
That would break current booting protocol in RISC-V where register "a0" should contain the booting hartid. If we have to move away for that method, changes need to be in multiple places (to modify the DT) and it has to be done in a backward compatible way.
Adding a new ABI between the firmware and the stub loader in Linux to use EFI specific conduits like config tables or EFI variables should really be avoided, though, as it affects every EFI loader while the code that runs in the EFI context doesn't even care (note that beyond u-boot and GRUB, there are other EFI loaders such as systemd-boot that need to be taken into account).
Which booting stage should be responsible for changing the DT for those EFI loaders ?