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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7A78AC433FE for ; Thu, 21 Oct 2021 14:33:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5870B61221 for ; Thu, 21 Oct 2021 14:33:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231450AbhJUOfZ (ORCPT ); Thu, 21 Oct 2021 10:35:25 -0400 Received: from foss.arm.com ([217.140.110.172]:43422 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231404AbhJUOfW (ORCPT ); Thu, 21 Oct 2021 10:35:22 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 90B06D6E; Thu, 21 Oct 2021 07:33:06 -0700 (PDT) Received: from [10.57.22.27] (unknown [10.57.22.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 524323F73D; Thu, 21 Oct 2021 07:33:04 -0700 (PDT) Subject: Re: [RFCv1 1/4] arm64: Use static key for tracing PID in CONTEXTIDR To: Leo Yan , Catalin Marinas , Will Deacon , Mark Rutland , Kees Cook , Ard Biesheuvel , Sami Tolvanen , Nicholas Piggin , James Morse , Marc Zyngier , Joey Gouly , Peter Collingbourne , Vincenzo Frascino , "Peter Zijlstra (Intel)" , Stephane Eranian , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20211021134530.206216-1-leo.yan@linaro.org> <20211021134530.206216-2-leo.yan@linaro.org> From: James Clark Message-ID: <53962765-53b9-dfdc-a5b2-a3133a924c12@arm.com> Date: Thu, 21 Oct 2021 15:33:01 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20211021134530.206216-2-leo.yan@linaro.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 21/10/2021 14:45, Leo Yan wrote: > The kernel provides CONFIG_PID_IN_CONTEXTIDR for tracing PID into system > register CONTEXTIDR; we need to statically enable this configuration > when build kernel image to use this feature. > > On the other hand, hardware tracing modules (e.g. Arm CoreSight, SPE, > etc) rely on this feature to provide context info in their tracing data. > If kernel has not enabled configuration CONFIG_PID_IN_CONTEXTIDR, then > tracing modules have no chance to capture PID related info. > > This patch introduces static key for tracing PID in CONTEXTIDR, it > provides a possibility for device driver to dynamically enable and > disable tracing PID into CONTEXTIDR as needed. > > As the first step, the kernel increases the static key if > CONFIG_PID_IN_CONTEXTIDR is enabled when booting kernel, in this case > kernel will always trace PID into CONTEXTIDR at the runtime. This means > before and after applying this patch, the semantics for > CONFIG_PID_IN_CONTEXTIDR are consistent. > > Signed-off-by: Leo Yan > --- > arch/arm64/include/asm/mmu_context.h | 4 +++- > arch/arm64/kernel/process.c | 11 +++++++++++ > 2 files changed, 14 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h > index f4ba93d4ffeb..e1f33616f83a 100644 > --- a/arch/arm64/include/asm/mmu_context.h > +++ b/arch/arm64/include/asm/mmu_context.h > @@ -26,9 +26,11 @@ > > extern bool rodata_full; > > +DECLARE_STATIC_KEY_FALSE(contextidr_in_use); > + > static inline void contextidr_thread_switch(struct task_struct *next) > { > - if (!IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR)) > + if (!static_branch_unlikely(&contextidr_in_use)) > return; > > write_sysreg(task_pid_nr(next), contextidr_el1); > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index 40adb8cdbf5a..d744c0c7e4c4 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -61,6 +61,9 @@ unsigned long __stack_chk_guard __ro_after_init; > EXPORT_SYMBOL(__stack_chk_guard); > #endif > > +DEFINE_STATIC_KEY_FALSE(contextidr_in_use); > +EXPORT_SYMBOL_GPL(contextidr_in_use); > + > /* > * Function pointers to optional machine specific functions > */ > @@ -721,3 +724,11 @@ int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state, > return prot; > } > #endif > + > +static int __init contextidr_init(void) > +{ > + if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR)) > + static_branch_inc(&contextidr_in_use); > + return 0; > +} > +early_initcall(contextidr_init); Hi Leo, Can you skip this early_initcall() part if you do something like this: DECLARE_STATIC_KEY_MAYBE(CONFIG_PID_IN_CONTEXTIDR, contextidr_in_use) It seems like there is a way to conditionally initialise it to true. James