[RFC PATCH 0/7] arm64: Make EFI calls preemptible

[Will Deacon]

Hey Ard,

On Wed, May 14, 2025 at 07:43:40PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> The arm64 port permits the use of the baseline FP/SIMD register file in
> kernel mode, and no longer requires preemption to be disabled. Now that
> the EFI spec is being clarified to state that EFI runtime services may
> only use baseline FP/SIMD, the fact that EFI may code may use FP/SIMD
> registers (while executing at the same privilege level as the kernel) is
> no longer a reason to disable preemption when invoking them.
> 
> This means that the only remaining reason for disabling preemption is
> the fact that the active mm is swapped out and replaced with efi_mm in a
> way that is hidden from the scheduler, and so scheduling is not
> supported currently. However, given that virtually all (*) EFI runtime
> calls are made from the efi_rts_wq workqueue, the efi_mm can simply be
> loaded into the workqueue worker kthread while the call is in progress,
> and this does not require preemption to be disabled.
> 
> Note that this is only a partial solution in terms of RT guarantees,
> given that the runtime services execute at the same privilege level as
> the kernel, and can therefore disable interrupts (and therefore
> preemption) directly. But it should prevent scheduling latency spikes
> for EFI calls that simply take a long time to run to completion.
> 
> (*) only efi_reset_system() and EFI pstore invoke EFI runtime services
>     without going through the workqueue, and the latter only when saving
>     a kernel oops log to the EFI varstore
> 
> Cc: Will Deacon <will at kernel.org>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
> Cc: Peter Zijlstra <peterz at infradead.org>
> 
> Ard Biesheuvel (7):
>   efi: Add missing static initializer for efi_mm::cpus_allowed_lock
>   efi/runtime: Return success/failure from arch_efi_call_virt_setup()
>   efi/runtime: Deal with arch_efi_call_virt_setup() returning failure
>   arm64/fpsimd: Don't warn when EFI execution context is preemptible
>   arm64/efi: Use a semaphore to protect the EFI stack and FP/SIMD state
>   arm64/efi: Move uaccess en/disable out of efi_set_pgd()
>   arm64/efi: Call EFI runtime services without disabling preemption

I quite like the idea of this from the arm64 side (modulo a small comment
I've left on one of the later patches), however it would be great to hear
from one of the -rt folk to confirm that this actually solves a problem
for them.

Will