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

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Message-ID: <aM1AqmHc7boSipIl@willie-the-truck>
Date: Fri, 19 Sep 2025 12:38:18 +0100
From: Will Deacon <will@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.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, Sep 18, 2025 at 01:33:48PM +0200, Ard Biesheuvel wrote:
> 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.

As discussed off-list, that's probably the best plan for now. I've left
a few small comments on some of the arm64 bits and it would be good to
give them some soak time in -next.

Cheers,

Will