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

[Ard Biesheuvel]

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

 arch/arm/include/asm/efi.h              |  2 +-
 arch/arm64/include/asm/efi.h            |  5 +--
 arch/arm64/kernel/efi.c                 | 34 ++++++++++++++++----
 arch/arm64/kernel/fpsimd.c              |  4 +--
 arch/loongarch/include/asm/efi.h        |  2 +-
 arch/riscv/include/asm/efi.h            |  2 +-
 arch/x86/include/asm/efi.h              |  2 +-
 arch/x86/platform/efi/efi_32.c          |  3 +-
 arch/x86/platform/efi/efi_64.c          |  3 +-
 arch/x86/platform/uv/bios_uv.c          |  3 +-
 drivers/firmware/efi/efi.c              |  3 ++
 drivers/firmware/efi/riscv-runtime.c    |  3 +-
 drivers/firmware/efi/runtime-wrappers.c | 20 ++++++++----
 include/linux/efi.h                     |  8 ++---
 14 files changed, 63 insertions(+), 31 deletions(-)


base-commit: e04796c8b5980700c78f2fd1b29724afd80dcc62
-- 
2.49.0.1101.gccaa498523-goog