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

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Message-ID: <CAMj1kXF1AAE9sLvDR0YwjZ0pDcnGG4cwqcy6VadUF-jOa7GkJw@mail.gmail.com>
Date: Thu, 18 Sep 2025 13:48:10 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Will Deacon <will@...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, 18 Sept 2025 at 13:44, Will Deacon <will@...nel.org> wrote:
>
> 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.
>
> I'm also happy to take the whole lot via arm64 this cycle, if you like?
> I reviewed it a while ago and was happy with it then.
>

I've made some major changes this time, so please double check that
you're still ok with it.

In particular, I've ripped out all of the special EFI handling in fpsimd.c