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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 8AC76C282CE for ; Thu, 11 Apr 2019 16:34:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E023217D4 for ; Thu, 11 Apr 2019 16:34:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726886AbfDKQee (ORCPT ); Thu, 11 Apr 2019 12:34:34 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:45974 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726564AbfDKQea (ORCPT ); Thu, 11 Apr 2019 12:34:30 -0400 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 CAEDE374; Thu, 11 Apr 2019 09:34:29 -0700 (PDT) Received: from e103592.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4E94C3F59C; Thu, 11 Apr 2019 09:34:28 -0700 (PDT) Date: Thu, 11 Apr 2019 17:34:25 +0100 From: Dave Martin To: Julien Grall Cc: linux-arm-kernel@lists.infradead.org, bigeasy@linutronix.de, linux-rt-users@vger.kernel.org, catalin.marinas@arm.com, will.deacon@arm.com, ard.biesheuvel@linaro.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state Message-ID: <20190411163423.GJ3567@e103592.cambridge.arm.com> References: <20190208165513.8435-1-julien.grall@arm.com> <20190404105233.GD3567@e103592.cambridge.arm.com> <20190405150722.GE3567@e103592.cambridge.arm.com> <899713e0-2d32-04a0-b2f2-3493f7299033@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <899713e0-2d32-04a0-b2f2-3493f7299033@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 11, 2019 at 04:58:41PM +0100, Julien Grall wrote: > Hi Dave, > > On 4/5/19 4:07 PM, Dave Martin wrote: > >On Fri, Apr 05, 2019 at 10:02:45AM +0100, Julien Grall wrote: > >>>>+#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); > >>> > >>>I find these names a bit confusing: _disable() actualy enables FPSIMD/SVE > >>>context access for the current context (i.e., makes it safe). > >>> > >>>Since these also disable/enable preemption, perhaps we can align them > >>>with the existing get_cpu()/put_cpu(), something like: > >>> > >>>void get_cpu_fpsimd_context(); > >>>vold put_cpu_fpsimd_context(); > >>> > >>>If you consider it's worth adding the checking helper I alluded to > >>>above, it could then be called something like: > >>> > >>>bool have_cpu_fpsimd_context(); > >> > >>I am not sure where you suggested a checking helper above. Do you refer to > >>exposing kernel_neon_busy even for !CONFIG_KERNEL_MODE_NEON? > > > >Hmmm, looks like I got my reply out of order here. > > > >I meant the helper (if any) to check > >!preemptible() && !__this_cpu_read(kernel_neon_busy). > > I guess you are using && instead of || because some of the caller may not > call get_cpu_fpsimd_context() before but still disable preemption, right? > > Wouldn't it be better to have all the user calling get_cpu_fpsimd_context() > and put_cpu_fpsimd_context()? > > This has the advantage to uniformize how the way FPSIMD is protected and > also... My expectation is that all users would have called get_cpu_fpsimd_context(). The reason for writing the check that way is that we can't meaningfully inspect percpu variables unless we are non-preemptible already. The && means we don't do the percpu read at all is the case where preemptible() is true. Or do you think my logic is wrong somewhere? (It's possible...) > >Looks like you inferred what I meant later on anyway. > > > >> > >>> > >>>>+ > >>>> /* > >>>> * 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. > >>> > >>>[*] That's not enough: we need to be in kernel_neon_disable()... > >>>_enable() (i.e., kernel_neon_busy needs to be true to prevent softirqs > >>>messing with the FPSIMD state). > >> > >>How about not mentioning preemption at all and just say: > >> > >>"The fpsimd context should be acquired before hand". > >> > >>This would help if we ever decide to protect critical section differently. > > > >Yes, or even better, name the function used to do this (i.e., > >kernel_neon_disable() or get_cpu_fpsimd_context() or whatever it's > >called). > > ... would make the comments simpler because we would have only one possible > case to care. Agreed ---Dave