From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D4B29C169C4 for ; Fri, 8 Feb 2019 17:03:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 98E9A2177B for ; Fri, 8 Feb 2019 17:03:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727915AbfBHRDk (ORCPT ); Fri, 8 Feb 2019 12:03:40 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:54176 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727816AbfBHRDY (ORCPT ); Fri, 8 Feb 2019 12:03:24 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id BE4F51596; Fri, 8 Feb 2019 09:03:23 -0800 (PST) Received: from e108454-lin.cambridge.arm.com (e108454-lin.cambridge.arm.com [10.1.196.50]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 014493F719; Fri, 8 Feb 2019 09:03:22 -0800 (PST) From: Julien Grall To: linux-kernel@vger.kernel.org Cc: Julien Grall Subject: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state Date: Fri, 8 Feb 2019 17:03:12 +0000 Message-Id: <20190208170315.10762-2-julien.grall@arm.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20190208170315.10762-1-julien.grall@arm.com> References: <20190208170315.10762-1-julien.grall@arm.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When the kernel is compiled with CONFIG_KERNEL_MODE_NEON, some part of the kernel may be able to use FPSIMD/SVE. This is for instance the case for crypto code. Any use of FPSIMD/SVE in the kernel are clearly marked by using the function kernel_neon_{begin, end}. Furthermore, this can only be used when may_use_simd() returns true. The current implementation of may_use_simd() allows softirq to use FPSIMD/SVE unless it is currently in used (i.e kernel_neon_busy is true). When in used, softirqs usually fallback to a software method. At the moment, as a softirq may use FPSIMD/SVE, softirqs are disabled when touching the FPSIMD/SVE context. This has the drawback to disable all softirqs even if they are not using FPSIMD/SVE. As a softirq should not rely on been able to use simd at a given time, there are limited reason to keep softirq disabled when touching the FPSIMD/SVE context. Instead, we can only disable preemption and tell the NEON unit is currently in use. This patch introduces two new helpers kernel_neon_{disable, enable} to mark the area using FPSIMD/SVE context and use them in replacement of local_bh_{disable, enable}. The functions kernel_neon_{begin, end} are also re-implemented to use the new helpers. Signed-off-by: Julien Grall --- I have been exploring this solution as an alternative approach to the RT patch "arm64: fpsimd: use preemp_disable in addition to local_bh_disable()". So far, the patch has only been lightly tested. For RT-linux, it might be possible to use migrate_{enable, disable}. I am quite new with RT and have some trouble to understand the semantics of migrate_{enable, disable}. So far, I am still unsure if it is possible to run another userspace task on the same CPU while getting preempted when the migration is disabled. --- arch/arm64/include/asm/simd.h | 4 +-- arch/arm64/kernel/fpsimd.c | 76 +++++++++++++++++++++++++------------------ 2 files changed, 46 insertions(+), 34 deletions(-) diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h index 6495cc51246f..94c0dac508aa 100644 --- a/arch/arm64/include/asm/simd.h +++ b/arch/arm64/include/asm/simd.h @@ -15,10 +15,10 @@ #include #include -#ifdef CONFIG_KERNEL_MODE_NEON - DECLARE_PER_CPU(bool, kernel_neon_busy); +#ifdef CONFIG_KERNEL_MODE_NEON + /* * may_use_simd - whether it is allowable at this time to issue SIMD * instructions or access the SIMD register file diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 5ebe73b69961..b7e5dac26190 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -90,7 +90,8 @@ * 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 local_bh_disable() unless softirqs are already masked. + * flag with kernel_neon_{disable, enable}. This will still allow softirqs to + * run but prevent them to use FPSIMD. * * For a certain task, the sequence may look something like this: * - the task gets scheduled in; if both the task's fpsimd_cpu field @@ -142,6 +143,9 @@ extern void __percpu *efi_sve_state; #endif /* ! CONFIG_ARM64_SVE */ +static void kernel_neon_disable(void); +static void kernel_neon_enable(void); + /* * Call __sve_free() directly only if you know task can't be scheduled * or preempted. @@ -213,11 +217,11 @@ static void sve_free(struct task_struct *task) * thread_struct is known to be up to date, when preparing to enter * userspace. * - * Softirqs (and preemption) must be disabled. + * Preemption must be disabled. */ static void task_fpsimd_load(void) { - WARN_ON(!in_softirq() && !irqs_disabled()); + WARN_ON(!preempt_count() && !irqs_disabled()); if (system_supports_sve() && test_thread_flag(TIF_SVE)) sve_load_state(sve_pffr(¤t->thread), @@ -238,7 +242,7 @@ void fpsimd_save(void) struct user_fpsimd_state *st = __this_cpu_read(fpsimd_last_state.st); /* set by fpsimd_bind_task_to_cpu() or fpsimd_bind_state_to_cpu() */ - WARN_ON(!in_softirq() && !irqs_disabled()); + WARN_ON(!preempt_count() && !irqs_disabled()); if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) { if (system_supports_sve() && test_thread_flag(TIF_SVE)) { @@ -360,7 +364,7 @@ static int __init sve_sysctl_init(void) { return 0; } * task->thread.sve_state. * * Task can be a non-runnable task, or current. In the latter case, - * softirqs (and preemption) must be disabled. + * preemption must be disabled. * task->thread.sve_state must point to at least sve_state_size(task) * bytes of allocated kernel memory. * task->thread.uw.fpsimd_state must be up to date before calling this @@ -387,7 +391,7 @@ static void fpsimd_to_sve(struct task_struct *task) * task->thread.uw.fpsimd_state. * * Task can be a non-runnable task, or current. In the latter case, - * softirqs (and preemption) must be disabled. + * preemption must be disabled. * task->thread.sve_state must point to at least sve_state_size(task) * bytes of allocated kernel memory. * task->thread.sve_state must be up to date before calling this function. @@ -547,7 +551,7 @@ int sve_set_vector_length(struct task_struct *task, * non-SVE thread. */ if (task == current) { - local_bh_disable(); + kernel_neon_disable(); fpsimd_save(); set_thread_flag(TIF_FOREIGN_FPSTATE); @@ -558,7 +562,7 @@ int sve_set_vector_length(struct task_struct *task, sve_to_fpsimd(task); if (task == current) - local_bh_enable(); + kernel_neon_enable(); /* * Force reallocation of task SVE state to the correct size @@ -813,7 +817,7 @@ asmlinkage void do_sve_acc(unsigned int esr, struct pt_regs *regs) sve_alloc(current); - local_bh_disable(); + kernel_neon_disable(); fpsimd_save(); fpsimd_to_sve(current); @@ -825,7 +829,7 @@ asmlinkage void do_sve_acc(unsigned int esr, struct pt_regs *regs) if (test_and_set_thread_flag(TIF_SVE)) WARN_ON(1); /* SVE access shouldn't have trapped */ - local_bh_enable(); + kernel_neon_enable(); } /* @@ -892,7 +896,7 @@ void fpsimd_flush_thread(void) if (!system_supports_fpsimd()) return; - local_bh_disable(); + kernel_neon_disable(); memset(¤t->thread.uw.fpsimd_state, 0, sizeof(current->thread.uw.fpsimd_state)); @@ -935,7 +939,7 @@ void fpsimd_flush_thread(void) set_thread_flag(TIF_FOREIGN_FPSTATE); - local_bh_enable(); + kernel_neon_enable(); } /* @@ -947,9 +951,9 @@ void fpsimd_preserve_current_state(void) if (!system_supports_fpsimd()) return; - local_bh_disable(); + kernel_neon_disable(); fpsimd_save(); - local_bh_enable(); + kernel_neon_enable(); } /* @@ -1007,14 +1011,14 @@ void fpsimd_restore_current_state(void) if (!system_supports_fpsimd()) return; - local_bh_disable(); + kernel_neon_disable(); if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) { task_fpsimd_load(); fpsimd_bind_task_to_cpu(); } - local_bh_enable(); + kernel_neon_enable(); } /* @@ -1027,7 +1031,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state) if (!system_supports_fpsimd()) return; - local_bh_disable(); + kernel_neon_disable(); current->thread.uw.fpsimd_state = *state; if (system_supports_sve() && test_thread_flag(TIF_SVE)) @@ -1038,7 +1042,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state) clear_thread_flag(TIF_FOREIGN_FPSTATE); - local_bh_enable(); + kernel_neon_enable(); } /* @@ -1055,11 +1059,28 @@ void fpsimd_flush_cpu_state(void) set_thread_flag(TIF_FOREIGN_FPSTATE); } -#ifdef CONFIG_KERNEL_MODE_NEON - DEFINE_PER_CPU(bool, kernel_neon_busy); EXPORT_PER_CPU_SYMBOL(kernel_neon_busy); +static void kernel_neon_disable(void) +{ + preempt_disable(); + WARN_ON(__this_cpu_read(kernel_neon_busy)); + __this_cpu_write(kernel_neon_busy, true); +} + +static void kernel_neon_enable(void) +{ + bool busy; + + busy = __this_cpu_xchg(kernel_neon_busy, false); + WARN_ON(!busy); /* No matching kernel_neon_disable()? */ + + preempt_enable(); +} + +#ifdef CONFIG_KERNEL_MODE_NEON + /* * Kernel-side NEON support functions */ @@ -1084,9 +1105,7 @@ void kernel_neon_begin(void) BUG_ON(!may_use_simd()); - local_bh_disable(); - - __this_cpu_write(kernel_neon_busy, true); + kernel_neon_disable(); /* Save unsaved fpsimd state, if any: */ fpsimd_save(); @@ -1094,9 +1113,7 @@ void kernel_neon_begin(void) /* Invalidate any task state remaining in the fpsimd regs: */ fpsimd_flush_cpu_state(); - preempt_disable(); - - local_bh_enable(); + kernel_neon_enable(); } EXPORT_SYMBOL(kernel_neon_begin); @@ -1111,15 +1128,10 @@ EXPORT_SYMBOL(kernel_neon_begin); */ void kernel_neon_end(void) { - bool busy; - if (!system_supports_fpsimd()) return; - busy = __this_cpu_xchg(kernel_neon_busy, false); - WARN_ON(!busy); /* No matching kernel_neon_begin()? */ - - preempt_enable(); + kernel_neon_enable(); } EXPORT_SYMBOL(kernel_neon_end); -- 2.11.0