[PATCH v2 1/1] efi_loader: architecture specific UEFI setup

[Chang, Abner (HPS SW/FW Technologist)]

> -----Original Message-----
> From: Atish Patra [mailto:atishp at atishpatra.org]
> Sent: Friday, February 7, 2020 6:56 AM
> To: Ard Biesheuvel <ard.biesheuvel at linaro.org>; Chang, Abner (HPS SW/FW
> Technologist) <abner.chang at hpe.com>
> Cc: Alexander Graf <agraf at csgraf.de>; Heinrich Schuchardt
> <xypron.glpk at gmx.de>; U-Boot Mailing List <u-boot at lists.denx.de>; Atish
> Patra <atish.patra at wdc.com>; leif at 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 at linaro.org>
> wrote:
> >
> > On Thu, 6 Feb 2020 at 21:06, Atish Patra <atishp at atishpatra.org> wrote:
> > >
> > > On Thu, Feb 6, 2020 at 12:10 PM Alexander Graf <agraf at 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 at linaro.org> wrote:
> > > > >> On Wed, 5 Feb 2020 at 05:53, Heinrich Schuchardt
> <xypron.glpk at 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 at gmx.de>
> > > > >>> Reviewed-by: Atish Patra <atish.patra at 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

Abner

> 
> Added Leif and Abner for the opinion.
> 
> 
> 
> --
> Regards,
> Atish