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=-16.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 1812EC433F5 for ; Thu, 9 Sep 2021 13:54:24 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id A6211611C7 for ; Thu, 9 Sep 2021 13:54:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org A6211611C7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvack.org Received: by kanga.kvack.org (Postfix) id 167356B006C; Thu, 9 Sep 2021 09:54:23 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 117A16B0072; Thu, 9 Sep 2021 09:54:23 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 00587900002; Thu, 9 Sep 2021 09:54:22 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0127.hostedemail.com [216.40.44.127]) by kanga.kvack.org (Postfix) with ESMTP id E1AA96B006C for ; Thu, 9 Sep 2021 09:54:22 -0400 (EDT) Received: from smtpin25.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 718A02D245 for ; Thu, 9 Sep 2021 13:54:22 +0000 (UTC) X-FDA: 78568179564.25.CF36849 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf02.hostedemail.com (Postfix) with ESMTP id 15B0C7001A0A for ; Thu, 9 Sep 2021 13:54:21 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id 96CFB61264 for ; Thu, 9 Sep 2021 13:54:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1631195660; bh=wxBTAJeOQ7MeLpizJg2FKExj67MUK1RXo6sqqLtc3Xo=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=acXhtfwZLRSW++s4vpRVkF8y9kGQij45KNFpPpoi3BnAZr4lrhG+Oy5Z2XS/AeRaN X7bYJVU48OIgxCgsocHrl9UDINq/5OP4QWaRB60wYgBNY3RlpZk9tIAOZSSs1tn5ts KpFBvjv4n59oqvyWlx+9Zxbn5dct054ZxnmbZP/yvqoTvMNb9uZ6VgqNlFVKutyNkn N2r2NLgKyZ3a9JzoxYettAzxVgHN3lzZtRMmD6EYiypbs3JQAlw+VjhLjhPK3GPMJF 5v/OZwiolP34hpPi1RYwrF1wtM2JJlColdtMRBh8nsVroLCCDA1xOoyS1uMBqP5bW2 ZVD4xKp4bRILg== Received: by mail-ot1-f54.google.com with SMTP id m7-20020a9d4c87000000b0051875f56b95so2561138otf.6 for ; Thu, 09 Sep 2021 06:54:20 -0700 (PDT) X-Gm-Message-State: AOAM5330qBuL214THIt6AeFTEWfra7oYNKg9gph427DvlhJ21DQ1IDsl qi7Ir42VQs7pCycZmtEo5D6i7SC11WnPjM3a1VE= X-Google-Smtp-Source: ABdhPJwTAIJ7a6Q8xfcCOYf16QFYVRJbMmyw75DbA9S8CvOrm7ZEqo6hO/7Ut+wiBW/tBTix5EXK4xui6c5FyG2orZY= X-Received: by 2002:a05:6830:603:: with SMTP id w3mr2540426oti.147.1631195659768; Thu, 09 Sep 2021 06:54:19 -0700 (PDT) MIME-Version: 1.0 References: <20210904060908.1310204-1-keithp@keithp.com> <20210907220038.91021-1-keithpac@amazon.com> <20210907220038.91021-6-keithpac@amazon.com> In-Reply-To: <20210907220038.91021-6-keithpac@amazon.com> From: Ard Biesheuvel Date: Thu, 9 Sep 2021 15:54:08 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 5/7] ARM: Stop using TPIDRPRW to hold per_cpu_offset To: Keith Packard Cc: Linux Kernel Mailing List , Abbott Liu , Andrew Morton , Andrey Ryabinin , Anshuman Khandual , Arnd Bergmann , Bjorn Andersson , Christoph Lameter , Dennis Zhou , Geert Uytterhoeven , Jens Axboe , Joe Perches , Kees Cook , Krzysztof Kozlowski , Linus Walleij , Linux ARM , Linux Memory Management List , Manivannan Sadhasivam , Marc Zyngier , Masahiro Yamada , Mike Rapoport , Nathan Chancellor , Nick Desaulniers , Nick Desaulniers , Nicolas Pitre , Russell King , Tejun Heo , Thomas Gleixner , =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= , Valentin Schneider , Viresh Kumar , "Wolfram Sang (Renesas)" , YiFei Zhu Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 15B0C7001A0A X-Stat-Signature: kqrzqst9xqk7xefbkbjmnxriiqm8spq7 Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=acXhtfwZ; spf=pass (imf02.hostedemail.com: domain of ardb@kernel.org designates 198.145.29.99 as permitted sender) smtp.mailfrom=ardb@kernel.org; dmarc=pass (policy=none) header.from=kernel.org X-HE-Tag: 1631195661-931804 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 8 Sept 2021 at 00:00, Keith Packard wrote: > > We're going to store TPIDRPRW here instead > ? > Signed-off-by: Keith Packard I'd much prefer to keep using TPIDIRPRW for the per-CPU offsets, and use the user space TLS register for current. There are several reasons for this: - arm64 does the same - as someone who still cares about ARM while many have moved on to arm64 or RISC-V, I am still trying to maintain parity between ARM and arm64 where possible. - efficiency: loading the per-CPU offset using a CPU id stored in memory, which is then used to index the per-CPU offsets array in memory adds two additional loads to every load/store of a per-CPU variable - 'current' usually does not change value under the code's feet, whereas per-CPU offsets might change at any time. Given the fact that the CPU offset load is visible to the compiler as a memory access, I suppose this should be safe, but I would still prefer per-CPU access to avoid going via current if possible. > --- > arch/arm/include/asm/percpu.h | 31 ------------------------------- > arch/arm/kernel/setup.c | 7 ------- > arch/arm/kernel/smp.c | 3 --- > 3 files changed, 41 deletions(-) > > diff --git a/arch/arm/include/asm/percpu.h b/arch/arm/include/asm/percpu.h > index e2fcb3cfd3de..eeafcd6a3e01 100644 > --- a/arch/arm/include/asm/percpu.h > +++ b/arch/arm/include/asm/percpu.h > @@ -7,37 +7,6 @@ > > register unsigned long current_stack_pointer asm ("sp"); > > -/* > - * Same as asm-generic/percpu.h, except that we store the per cpu offset > - * in the TPIDRPRW. TPIDRPRW only exists on V6K and V7 > - */ > -#if defined(CONFIG_SMP) && !defined(CONFIG_CPU_V6) > -static inline void set_my_cpu_offset(unsigned long off) > -{ > - /* Set TPIDRPRW */ > - asm volatile("mcr p15, 0, %0, c13, c0, 4" : : "r" (off) : "memory"); > -} > - > -static inline unsigned long __my_cpu_offset(void) > -{ > - unsigned long off; > - > - /* > - * Read TPIDRPRW. > - * We want to allow caching the value, so avoid using volatile and > - * instead use a fake stack read to hazard against barrier(). > - */ > - asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) > - : "Q" (*(const unsigned long *)current_stack_pointer)); > - > - return off; > -} > -#define __my_cpu_offset __my_cpu_offset() > -#else > -#define set_my_cpu_offset(x) do {} while(0) > - > -#endif /* CONFIG_SMP */ > - > #include > > #endif /* _ASM_ARM_PERCPU_H_ */ > diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c > index ca0201635fac..d0dc60afe54f 100644 > --- a/arch/arm/kernel/setup.c > +++ b/arch/arm/kernel/setup.c > @@ -590,13 +590,6 @@ void __init smp_setup_processor_id(void) > for (i = 1; i < nr_cpu_ids; ++i) > cpu_logical_map(i) = i == cpu ? 0 : i; > > - /* > - * clear __my_cpu_offset on boot CPU to avoid hang caused by > - * using percpu variable early, for example, lockdep will > - * access percpu variable inside lock_release > - */ > - set_my_cpu_offset(0); > - > pr_info("Booting Linux on physical CPU 0x%x\n", mpidr); > } > > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c > index 5e999f1f1aea..8ccf10b34f08 100644 > --- a/arch/arm/kernel/smp.c > +++ b/arch/arm/kernel/smp.c > @@ -410,8 +410,6 @@ asmlinkage void secondary_start_kernel(unsigned int cpu, struct task_struct *tas > { > struct mm_struct *mm = &init_mm; > > - set_my_cpu_offset(per_cpu_offset(cpu)); > - > secondary_biglittle_init(); > > /* > @@ -495,7 +493,6 @@ void __init smp_cpus_done(unsigned int max_cpus) > > void __init smp_prepare_boot_cpu(void) > { > - set_my_cpu_offset(per_cpu_offset(smp_processor_id())); > } > > void __init smp_prepare_cpus(unsigned int max_cpus) > -- > 2.33.0 >