linux-kernel - Re: [PATCH v3 0/8] arm64: Make EFI calls preemptible

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Message-ID: <CAMj1kXGndZxxe+u3st6FCebrdutUibspA7tJUpqGMgpj9UTFnA@mail.gmail.com>
Date: Thu, 18 Sep 2025 13:33:48 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, Will Deacon <will@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Catalin Marinas <catalin.marinas@....com>, 
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v3 0/8] arm64: Make EFI calls preemptible

On Thu, 18 Sept 2025 at 12:30, Ard Biesheuvel <ardb+git@...gle.com> wrote:
>
> From: Ard Biesheuvel <ardb@...nel.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.
>
> Changes since v2:
> - Permit ordinary kernel mode FP/SIMD with IRQs disabled, so that the
>   special EFI case only deals with invocations in hardirq or NMI context
> - Disallow EFI runtime calls in hardirq or NMI context, so that the
>   special FP/SIMD handling for EFI can be dropped entirely
> - Use a mutex rather than a semaphore for the arm64 EFI runtime lock,
>   now that it is never trylock()ed in IRQ or NMI context.
>
> Changes since v1/RFC:
> - Disable uaccess for SWPAN before updating the preserved TTBR0 value
> - Document why disabling migration is needed
> - Rebase onto v6.17-rc1
>
> (*) 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@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Mark Brown <broonie@...nel.org>
>
> Ard Biesheuvel (8):
>   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

Unless anyone objects, I am going to queue up these 3 patches ^^^ via
the EFI tree.

>   arm64/fpsimd: Permit kernel mode NEON with IRQs off
>   arm64/fpsimd: Drop special handling for EFI runtime services
>   arm64/efi: Use a mutex 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
>

... so the rest can go in via the arm64 tree in the next cycle.