Re: [PATCH RFC 0/6] PSCI-via-EFI to support firmware and kernel sharing EL2 for Apple Silicon

["Ard Biesheuvel" <ardb@kernel.org>]

Hi Sven,

On Wed, 8 Jul 2026, at 09:15, Sven Peter wrote:
> Hi,
>
> Usually, idle and sleep state are implemented in firmware running in
> e.g. EL3 with the kernel trapping into that from EL2. Unfortunately,
> there's no EL3 on Apple Silicon machines and we'd rather not run the
> kernel in EL1 since this would result in losing KVM support.
>
> While the shallower states could be implemented inside a custom cpuidle
> driver (like we do downstream, see [1]) the deeper states result in a
> complete loss of state and require bootstraping the cores again which is
> quite involved. So instead we need some way to call back into our
> open-source firmware to be able to handle that. This is even more
> important for M4+ which don't even support the architectural wfi anymore
> and always lose state when that instruction is executed.
>
> Luckily, EFI runtime services provide much of scaffolding we need,
> namely a way to keep some code and data mapped and the ability to jump
> into there from inside the kernel.
>
> This series adds a custom EFI table that points to a PSCI entry point
> (plus some other stuff that has to be available before EFI runtime
> services are set up) and adds support for this new conduit to the psci
> code. We can't directly use the normal EFI runtime path because that one
> takes a sleeping lock and we need to be able to call into PSCI from
> atomic context during e.g. cpu bringup or during idle.
> It also adds support for specifying the specific MAIR attributes for EFI
> runtime mappings as defined in the latest UEFI spec since Apple Silicon
> is rather allergic to using Device-nGnRnE vs. Device-nGnRE for its MMIO.
>
> This all results in a surprisingly small diffstat. I believe this
> approach was originally suggested in some IRC discussion years ago,
> possibly by Ard, but I can't find the old logs anymore.
> Happy to add a Suggested-by tag though if anyone remembers.
>
> The firmware implementation I used for testing can be found at [2] and
> the full kernel tree with this series applied at [3].
>
> Best,
>
> Sven
>
> [1] 
> https://github.com/AsahiLinux/linux/blob/asahi/drivers/cpuidle/cpuidle-apple.c
> [2] https://github.com/AsahiLinux/m1n1/tree/psci-via-efi
> [3] 
> https://git.kernel.org/pub/scm/linux/kernel/git/sven/linux.git/log/?h=efi-psci
>
> Signed-off-by: Sven Peter <sven@kernel.org>
> ---
> Sven Peter (6):
>       dt-bindings: arm: psci: Add EFI conduit
>       arm64/efi: Add and parse custom PSCI EFI configuration table
>       efi: Add EFI_MEMORY_ISA_{MASK,VALID}
>       arm64/efi: Honor EFI_MEMORY_ISA_MASK for Device-nGnRnE vs -nGnRE
>       firmware/psci: Add EFI runtime conduit
>       arm64: dts: apple: t8103: Add PSCI and CPU idle states
>

I've picked up patches #3 and #4, which are useful in their own right.

I'm not sure if the arm64 maintainers will want to consider this, but
I think it's a reasonable compromise, as it puts the abstraction in
the right place.

I take it this will be contributed to u-boot if it gets accepted here?