Re: [PATCH v3 8/8] arm64/efi: Call EFI runtime services without disabling preemption

[Will Deacon <will@kernel.org>]

On Thu, Sep 18, 2025 at 12:30:19PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The only remaining reason why EFI runtime services are invoked with
> preemption disabled is the fact that the mm is swapped out behind the
> back of the context switching code.
> 
> The kernel no longer disables preemption in kernel_neon_begin().
> Furthermore, the EFI spec is being clarified to explicitly state that
> only baseline FP/SIMD is permitted in EFI runtime service
> implementations, and so the existing kernel mode NEON context switching
> code is sufficient to preserve and restore the execution context of an
> in-progress EFI runtime service call.
> 
> Most EFI calls are made from the efi_rts_wq, which is serviced by a
> kthread. As kthreads never return to user space, they usually don't have
> an mm, and so we can use the existing infrastructure to swap in the
> efi_mm while the EFI call is in progress. This is visible to the
> scheduler, which will therefore reactivate the selected mm when
> switching out the kthread and back in again.
> 
> Given that the EFI spec explicitly permits runtime services to be called
> with interrupts enabled, firmware code is already required to tolerate
> interruptions. So rather than disable preemption, disable only migration
> so that EFI runtime services are less likely to cause scheduling delays.
> To avoid potential issues where runtime services are interrupted while
> polling the secure firmware for async completions, keep migration
> disabled so that a runtime service invocation does not resume on a
> different CPU from the one it was started on.
> 
> Note, though, that the firmware executes at the same privilege level as
> the kernel, and is therefore able to disable interrupts altogether.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/kernel/efi.c | 23 ++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)

Acked-by: Will Deacon <will@kernel.org>

Will