From 7137a203b2515bdbeae1cf13446bdce17db2c2f7 Mon Sep 17 00:00:00 2001 From: Ard Biesheuvel Date: Wed, 15 Oct 2025 22:56:39 +0200 Subject: [PATCH] arm64/fpsimd: Permit kernel mode NEON with IRQs off 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(). Acked-by: Will Deacon Signed-off-by: Ard Biesheuvel Acked-by: Catalin Marinas Signed-off-by: Catalin Marinas --- arch/arm64/include/asm/simd.h | 2 +- arch/arm64/kernel/fpsimd.c | 25 +++++++++++++++++++------ 2 files changed, 20 insertions(+), 7 deletions(-) diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h index 8e86c9e70e483..abd642c92f863 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 3d848c89604ee..6d956ed23bd21 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -225,10 +225,21 @@ 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)) { + /* + * The softirq subsystem lacks a true unmask/mask API, and + * re-enabling softirq processing using local_bh_enable() will + * not only unmask softirqs, it will also result in immediate + * delivery of any pending softirqs. + * This is undesirable when running with IRQs disabled, but in + * that case, there is no need to mask softirqs in the first + * place, so only bother doing so when IRQs are enabled. + */ + if (!irqs_disabled()) + local_bh_disable(); + } else { preempt_disable(); + } } /* @@ -240,10 +251,12 @@ 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(); + } else { preempt_enable(); + } } unsigned int task_get_vl(const struct task_struct *task, enum vec_type type) -- 2.47.3