[v4,1/4] arm64: fpsimd: Drop unneeded 'busy' flag

Message ID	20231208113218.3001940-7-ardb@google.com
State	New
Headers	show Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="0JMNUNOx" Date: Fri, 8 Dec 2023 12:32:20 +0100 In-Reply-To: <20231208113218.3001940-6-ardb@google.com> Precedence: bulk Mime-Version: 1.0 References: <20231208113218.3001940-6-ardb@google.com> Message-ID: <20231208113218.3001940-7-ardb@google.com> Subject: [PATCH v4 1/4] arm64: fpsimd: Drop unneeded 'busy' flag From: Ard Biesheuvel <ardb@google.com> To: linux-arm-kernel@lists.infradead.org Cc: linux-crypto@vger.kernel.org, Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>, Mark Rutland <mark.rutland@arm.com>, Kees Cook <keescook@chromium.org>, Catalin Marinas <catalin.marinas@arm.com>, Mark Brown <broonie@kernel.org>, Eric Biggers <ebiggers@google.com>, Sebastian Andrzej Siewior <bigeasy@linutronix.de> Content-Type: text/plain; charset="UTF-8"
Series	arm64: Run kernel mode NEON with preemption enabled \| expand [v4,0/4] arm64: Run kernel mode NEON with preemption enabled [v4,1/4] arm64: fpsimd: Drop unneeded 'busy' flag [v4,2/4] arm64: fpsimd: Preserve/restore kernel mode NEON at context switch [v4,3/4] arm64: fpsimd: Implement lazy restore for kernel mode FPSIMD [v4,4/4] arm64: crypto: Disable yielding logic unless CONFIG_PREEMPT_VOLUNTARY=y

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h index 6a75d7ecdcaa..8e86c9e70e48 100644 --- a/arch/arm64/include/asm/simd.h +++ b/arch/arm64/include/asm/simd.h @@ -12,8 +12,6 @@ #include <linux/preempt.h> #include <linux/types.h> -DECLARE_PER_CPU(bool, fpsimd_context_busy); - #ifdef CONFIG_KERNEL_MODE_NEON /* @@ -28,17 +26,10 @@ static __must_check inline bool may_use_simd(void) /* * We must make sure that the SVE has been initialized properly * before using the SIMD in kernel. - * fpsimd_context_busy is only set while preemption is disabled, - * and is clear whenever preemption is enabled. Since - * this_cpu_read() is atomic w.r.t. preemption, fpsimd_context_busy - * cannot change under our feet -- if it's set we cannot be - * migrated, and if it's clear we cannot be migrated to a CPU - * where it is set. */ return !WARN_ON(!system_capabilities_finalized()) && system_supports_fpsimd() && - !in_hardirq() && !irqs_disabled() && !in_nmi() && - !this_cpu_read(fpsimd_context_busy); + !in_hardirq() && !irqs_disabled() && !in_nmi(); } #else /* ! CONFIG_KERNEL_MODE_NEON */ diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 1559c706d32d..ccc4a78a70e4 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -85,13 +85,13 @@ * softirq kicks in. Upon vcpu_put(), KVM will save the vcpu FP state and * flag the register state as invalid. * - * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may - * save the task's FPSIMD context back to task_struct from softirq context. - * To prevent this from racing with the manipulation of the task's FPSIMD state - * from task context and thereby corrupting the state, it is necessary to - * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE - * flag with {, __}get_cpu_fpsimd_context(). This will still allow softirqs to - * run but prevent them to use FPSIMD. + * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may be + * called from softirq context, which will save the task's FPSIMD context back + * to task_struct. To prevent this from racing with the manipulation of the + * task's FPSIMD state from task context and thereby corrupting the state, it + * is necessary to protect any manipulation of a task's fpsimd_state or + * TIF_FOREIGN_FPSTATE flag with get_cpu_fpsimd_context(), which will suspend + * softirq servicing entirely until put_cpu_fpsimd_context() is called. * * For a certain task, the sequence may look something like this: * - the task gets scheduled in; if both the task's fpsimd_cpu field @@ -209,27 +209,14 @@ static inline void sme_free(struct task_struct *t) { } #endif -DEFINE_PER_CPU(bool, fpsimd_context_busy); -EXPORT_PER_CPU_SYMBOL(fpsimd_context_busy); - static void fpsimd_bind_task_to_cpu(void); -static void __get_cpu_fpsimd_context(void) -{ - bool busy = __this_cpu_xchg(fpsimd_context_busy, true); - - WARN_ON(busy); -} - /* * Claim ownership of the CPU FPSIMD context for use by the calling context. * * The caller may freely manipulate the FPSIMD context metadata until * put_cpu_fpsimd_context() is called. * - * The double-underscore version must only be called if you know the task - * can't be preempted. - * * On RT kernels local_bh_disable() is not sufficient because it only * serializes soft interrupt related sections via a local lock, but stays * preemptible. Disabling preemption is the right choice here as bottom @@ -242,14 +229,6 @@ static void get_cpu_fpsimd_context(void) local_bh_disable(); else preempt_disable(); - __get_cpu_fpsimd_context(); -} - -static void __put_cpu_fpsimd_context(void) -{ - bool busy = __this_cpu_xchg(fpsimd_context_busy, false); - - WARN_ON(!busy); /* No matching get_cpu_fpsimd_context()? */ } /* @@ -261,18 +240,12 @@ static void __put_cpu_fpsimd_context(void) */ static void put_cpu_fpsimd_context(void) { - __put_cpu_fpsimd_context(); if (!IS_ENABLED(CONFIG_PREEMPT_RT)) local_bh_enable(); else preempt_enable(); } -static bool have_cpu_fpsimd_context(void) -{ - return !preemptible() && __this_cpu_read(fpsimd_context_busy); -} - unsigned int task_get_vl(const struct task_struct *task, enum vec_type type) { return task->thread.vl[type]; @@ -383,7 +356,7 @@ static void task_fpsimd_load(void) bool restore_ffr; WARN_ON(!system_supports_fpsimd()); - WARN_ON(!have_cpu_fpsimd_context()); + WARN_ON(preemptible()); if (system_supports_sve() || system_supports_sme()) { switch (current->thread.fp_type) { @@ -467,7 +440,7 @@ static void fpsimd_save(void) unsigned int vl; WARN_ON(!system_supports_fpsimd()); - WARN_ON(!have_cpu_fpsimd_context()); + WARN_ON(preemptible()); if (test_thread_flag(TIF_FOREIGN_FPSTATE)) return; @@ -1507,7 +1480,7 @@ void fpsimd_thread_switch(struct task_struct *next) if (!system_supports_fpsimd()) return; - __get_cpu_fpsimd_context(); + WARN_ON_ONCE(!irqs_disabled()); /* Save unsaved fpsimd state, if any: */ fpsimd_save(); @@ -1523,8 +1496,6 @@ void fpsimd_thread_switch(struct task_struct *next) update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE, wrong_task || wrong_cpu); - - __put_cpu_fpsimd_context(); } static void fpsimd_flush_thread_vl(enum vec_type type) @@ -1829,10 +1800,10 @@ void fpsimd_save_and_flush_cpu_state(void) if (!system_supports_fpsimd()) return; WARN_ON(preemptible()); - __get_cpu_fpsimd_context(); + get_cpu_fpsimd_context(); fpsimd_save(); fpsimd_flush_cpu_state(); - __put_cpu_fpsimd_context(); + put_cpu_fpsimd_context(); } #ifdef CONFIG_KERNEL_MODE_NEON

[v4,1/4] arm64: fpsimd: Drop unneeded 'busy' flag

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