Driver model at UEFI runtime

[Simon Glass]

Hi Heinrich,

On Thu, 9 Sept 2021 at 05:29, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> Hello Simon,
>
> The EBBR specification requires that the UEFI SystemReset() runtime
> service is available to the operating system.
>
> Up to now this has been implemented by overriding function
> efi_reset_system() which is marked as __efi_runtime.
>
> Both ARM and RISC-V support generic interfaces for reset. PSCI for ARM
> and the System Reset Extension for SBI on RISC-V. This has kept the
> number of implementations low. The only exceptions are:
>
> * arch/arm/cpu/armv8/fsl-layerscape/cpu.c
> * arch/arm/mach-bcm283x/reset.c for the Raspberry PIs
> * arch/sandbox/cpu/start.c
>
> Bin has suggested in
> https://lists.denx.de/pipermail/u-boot/2021-September/459865.html to use
> reset drivers based on the driver model.
>
> Currently after ExitBootServices() the driver model does not exist anymore.
>
> When evaluating Bin's suggestion one has to keep in mind that after
> invoking ExitBootServices() most operating systems call
> SetVirtualAddressMap(). Due to the change of the address map all
> pointers used by U-Boot afterwards must be updated to match the new
> memory map.
>
> The impression that Ilias and I have is that keeping the driver model
> alive after SetVirtualAddressMap() would incur:
>
> * a high development effort
> * a significant code size increase
> * an enlarged attack surface
>
> For RISC-V it has been clarified in the platform specification that the
> SBI must implement the system reset extension. For ARMv8 using TF-A and
> PSCI is what ARM suggests.
>
> So for these architectures we do not expect a growth in the number of
> drivers needed.
>
> Ilias and my favorite would be keeping the design as is.
>
> What is your view on this?

Not to dump on the original author but here again we are paying the
price for the shortcuts taken at the time and not since revisited.

My original request then was to create a new build of U-Boot, since we
need to build (and load) the runtime stuff separately. Then we can
avoid all this mess and just use the normal U-Boot code (and driver
model). It also scales up to whatever else we want to do to the untime
stuff in the future.

This will be somewhat easier with the VPL series applied, and even
easier if we can make the different builds more easily configurable.
TBD on that and Tom has suggested an approach with CONFIG options that
should tidy things up.

Of course the 'tools' builds solve this by copying/symlinking the C
files into a different directory so they can be built again in a
different context. I think that could get quite out of hand though. So
I favour a separate build.

Regards,
Simon