[PATCH v3 4/8] arm64/fpsimd: Permit kernel mode NEON with IRQs off

[Will Deacon]

On Thu, Sep 18, 2025 at 12:30:15PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> Currently, may_use_simd() will return false when called from a context
> where IRQs are disabled. One notable case where this happens is when
> calling the ResetSystem() EFI runtime service from the reboot/poweroff
> code path. For this case alone, there is a substantial amount of FP/SIMD
> support code to handle the corner case where a EFI runtime service is
> invoked with IRQs disabled.
> 
> The only reason kernel mode SIMD is not allowed when IRQs are disabled
> is that re-enabling softirqs in this case produces a noisy diagnostic
> when lockdep is enabled. The warning is valid, in the sense that
> delivering pending softirqs over the back of the call to
> local_bh_enable() is problematic when IRQs are disabled.
> 
> While the API lacks a facility to simply mask and unmask softirqs
> without triggering their delivery, disabling softirqs is not needed to
> begin with when IRQs are disabled, given that softirqs are only every
> taken asynchronously over the back of a hard IRQ.
> 
> So dis/enable softirq processing conditionally, based on whether IRQs
> are enabled, and relax the check in may_use_simd().
> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm64/include/asm/simd.h |  2 +-
>  arch/arm64/kernel/fpsimd.c    | 16 ++++++++++------
>  2 files changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> index 8e86c9e70e48..abd642c92f86 100644
> --- a/arch/arm64/include/asm/simd.h
> +++ b/arch/arm64/include/asm/simd.h
> @@ -29,7 +29,7 @@ static __must_check inline bool may_use_simd(void)
>  	 */
>  	return !WARN_ON(!system_capabilities_finalized()) &&
>  	       system_supports_fpsimd() &&
> -	       !in_hardirq() && !irqs_disabled() && !in_nmi();
> +	       !in_hardirq() && !in_nmi();
>  }
>  
>  #else /* ! CONFIG_KERNEL_MODE_NEON */
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index c37f02d7194e..96a226316d1f 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -225,10 +225,12 @@ static void fpsimd_bind_task_to_cpu(void);
>   */
>  static void get_cpu_fpsimd_context(void)
>  {
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> -		local_bh_disable();
> -	else
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		if (!irqs_disabled())
> +			local_bh_disable();
> +	} else {
>  		preempt_disable();
> +	}
>  }
>  
>  /*
> @@ -240,10 +242,12 @@ static void get_cpu_fpsimd_context(void)
>   */
>  static void put_cpu_fpsimd_context(void)
>  {
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> -		local_bh_enable();
> -	else
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		if (!irqs_disabled())
> +			local_bh_enable();

This is a little ugly, so I think a small comment summarising the
penultimate paragraph of your commit message would help to explain the dance
you're doing.

With that:

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

Will