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=-19.4 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,USER_AGENT_GIT 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 3DACFC43460 for ; Tue, 18 May 2021 09:49:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1ED8F613F2 for ; Tue, 18 May 2021 09:49:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348151AbhERJvG (ORCPT ); Tue, 18 May 2021 05:51:06 -0400 Received: from mail.kernel.org ([198.145.29.99]:50270 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348147AbhERJu3 (ORCPT ); Tue, 18 May 2021 05:50:29 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 8011561404; Tue, 18 May 2021 09:48:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1621331332; bh=cLV1ruPvNZQWbLhrPTW5PTi59kS93zO0HxNZ8xc2n5I=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=PpUmRDuQPotlvWn6Zdm8S/5OhittrHyEVMvHp0uVcz4ZQfZit07yzmo/gvmt5fJsE cANOh+AlooAZYy7DCWLxMZulfgk9zCvL47X8Tn92OpSXCSPJeD3/tzsPsoTZXrUQkC vqHcsROicuUlVI0xVzlbqTLW7e+1kAKB8hVDeVnGS1ZX22G1ESF0XRx9ExjyuyR1OM gvoxgDN/VeMMK2hlyigf3JRWaHnKV60msIvhPv6RakToZXn5bVmQZpnB46ht3jbu8Z 5C6SSpXjuLXDH9n7Hwurn7z7rPVcR4gNK9M0rCfL/1BZha1gy5IxmVW1IvUiZCR5YY CpM3nGLV19GbQ== From: Will Deacon To: linux-arm-kernel@lists.infradead.org Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Will Deacon , Catalin Marinas , Marc Zyngier , Greg Kroah-Hartman , Peter Zijlstra , Morten Rasmussen , Qais Yousef , Suren Baghdasaryan , Quentin Perret , Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , "Rafael J. Wysocki" , kernel-team@android.com Subject: [PATCH v6 21/21] Documentation: arm64: describe asymmetric 32-bit support Date: Tue, 18 May 2021 10:47:25 +0100 Message-Id: <20210518094725.7701-22-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20210518094725.7701-1-will@kernel.org> References: <20210518094725.7701-1-will@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Document support for running 32-bit tasks on asymmetric 32-bit systems and its impact on the user ABI when enabled. Signed-off-by: Will Deacon --- .../admin-guide/kernel-parameters.txt | 3 + Documentation/arm64/asymmetric-32bit.rst | 149 ++++++++++++++++++ Documentation/arm64/index.rst | 1 + 3 files changed, 153 insertions(+) create mode 100644 Documentation/arm64/asymmetric-32bit.rst diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index a2e453919bb6..5a1dc7e628a5 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -295,6 +295,9 @@ EL0 is indicated by /sys/devices/system/cpu/aarch32_el0 and hot-unplug operations may be restricted. + See Documentation/arm64/asymmetric-32bit.rst for more + information. + amd_iommu= [HW,X86-64] Pass parameters to the AMD IOMMU driver in the system. Possible values are: diff --git a/Documentation/arm64/asymmetric-32bit.rst b/Documentation/arm64/asymmetric-32bit.rst new file mode 100644 index 000000000000..baf02c143363 --- /dev/null +++ b/Documentation/arm64/asymmetric-32bit.rst @@ -0,0 +1,149 @@ +====================== +Asymmetric 32-bit SoCs +====================== + +Author: Will Deacon + +This document describes the impact of asymmetric 32-bit SoCs on the +execution of 32-bit (``AArch32``) applications. + +Date: 2021-05-17 + +Introduction +============ + +Some Armv9 SoCs suffer from a big.LITTLE misfeature where only a subset +of the CPUs are capable of executing 32-bit user applications. On such +a system, Linux by default treats the asymmetry as a "mismatch" and +disables support for both the ``PER_LINUX32`` personality and +``execve(2)`` of 32-bit ELF binaries, with the latter returning +``-ENOEXEC``. If the mismatch is detected during late onlining of a +64-bit-only CPU, then the onlining operation fails and the new CPU is +unavailable for scheduling. + +Surprisingly, these SoCs have been produced with the intention of +running legacy 32-bit binaries. Unsurprisingly, that doesn't work very +well with the default behaviour of Linux. + +It seems inevitable that future SoCs will drop 32-bit support +altogether, so if you're stuck in the unenviable position of needing to +run 32-bit code on one of these transitionary platforms then you would +be wise to consider alternatives such as recompilation, emulation or +retirement. If neither of those options are practical, then read on. + +Enabling kernel support +======================= + +Since the kernel support is not completely transparent to userspace, +allowing 32-bit tasks to run on an asymmetric 32-bit system requires an +explicit "opt-in" and can be enabled by passing the +``allow_mismatched_32bit_el0`` parameter on the kernel command-line. + +For the remainder of this document we will refer to an *asymmetric +system* to mean an SoC running Linux with this kernel command-line +option enabled. + +Userspace impact +================ + +32-bit tasks running on an asymmetric system behave in mostly the same +way as on a homogeneous system, with a few key differences relating to +CPU affinity. + +sysfs +----- + +The subset of CPUs capable of running 32-bit tasks is described in +``/sys/devices/system/cpu/aarch32_el0`` and is documented further in +``Documentation/ABI/testing/sysfs-devices-system-cpu``. + +**Note:** CPUs are advertised by this file as they are detected and so +late-onlining of 32-bit-capable CPUs can result in the file contents +being modified by the kernel at runtime. Once advertised, CPUs are never +removed from the file. + +``execve(2)`` +------------- + +On a homogeneous system, the CPU affinity of a task is preserved across +``execve(2)``. This is not always possible on an asymmetric system, +specifically when the new program being executed is 32-bit yet the +affinity mask contains 64-bit-only CPUs. In this situation, the kernel +determines the new affinity mask as follows: + + 1. If the 32-bit-capable subset of the affinity mask is not empty, + then the affinity is restricted to that subset and the old affinity + mask is saved. This saved mask is inherited over ``fork(2)`` and + preserved across ``execve(2)`` of 32-bit programs. + + **Note:** This step does not apply to ``SCHED_DEADLINE`` tasks. + See `SCHED_DEADLINE`_. + + 2. Otherwise, the cpuset hierarchy of the task is walked until an + ancestor is found containing at least one 32-bit-capable CPU. The + affinity of the task is then changed to match the 32-bit-capable + subset of the cpuset determined by the walk. + + 3. On failure (i.e. out of memory), the affinity is changed to the set + of all 32-bit-capable CPUs of which the kernel is aware. + +A subsequent ``execve(2)`` of a 64-bit program by the 32-bit task will +invalidate the affinity mask saved in (1) and attempt to restore the CPU +affinity of the task using the saved mask if it was previously valid. +This restoration may fail due to intervening changes to the deadline +policy or cpuset hierarchy, in which case the ``execve(2)`` continues +with the affinity unchanged. + +Calls to ``sched_setaffinity(2)`` for a 32-bit task will consider only +the 32-bit-capable CPUs of the requested affinity mask. On success, the +affinity for the task is updated and any saved mask from a prior +``execve(2)`` is invalidated. + +``SCHED_DEADLINE`` +------------------ + +Admitting a 32-bit task to the deadline scheduler (e.g. by calling +``sched_setattr(2)``) will, if valid, consider the affinity mask saved +by a previous call to ``execve(2)`` for the purposes of input validation +in preference to the running affinity of the task. 64-bit deadline tasks +will skip step (1) of the process described in `execve(2)`_ when +executed a 32-bit program. + +**Note:** It is recommended that the 32-bit-capable CPUs are placed into +a separate root domain if ``SCHED_DEADLINE`` is to be used with 32-bit +tasks on an asymmetric system. Failure to do so is likely to result in +missed deadlines. + +Cpusets +------- + +The affinity of a 32-bit task may include CPUs that are not explicitly +allowed by the cpuset to which it is attached. This can occur as a +result of the following two situations: + + - A 64-bit task attached to a cpuset which allows only 64-bit CPUs + executes a 32-bit program. + + - All of the 32-bit-capable CPUs allowed by a cpuset containing a + 32-bit task are offlined. + +In both of these cases, the new affinity is calculated according to step +(2) of the process described in `execve(2)`_ and the cpuset hierarchy is +unchanged irrespective of the cgroup version. + +CPU hotplug +----------- + +When the kernel detects asymmetric 32-bit hardware, the first detected +32-bit-capable CPU is prevented from being offlined by userspace and any +such attempt will return ``-EPERM``. Note that suspend is still +permitted even if the primary CPU (i.e. CPU 0) is 64-bit-only. + +KVM +--- + +Although KVM will not advertise 32-bit EL0 support to any vCPUs on an +asymmetric system, a broken guest at EL1 could still attempt to execute +32-bit code at EL0. In this case, an exit from a vCPU thread in 32-bit +mode will return to host userspace with an ``exit_reason`` of +``KVM_EXIT_FAIL_ENTRY``. diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst index 97d65ba12a35..4f840bac083e 100644 --- a/Documentation/arm64/index.rst +++ b/Documentation/arm64/index.rst @@ -10,6 +10,7 @@ ARM64 Architecture acpi_object_usage amu arm-acpi + asymmetric-32bit booting cpu-feature-registers elf_hwcaps -- 2.31.1.751.gd2f1c929bd-goog 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=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 96417C43460 for ; Tue, 18 May 2021 10:12:23 +0000 (UTC) Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1B06F61185 for ; Tue, 18 May 2021 10:12:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1B06F61185 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=desiato.20200630; h=Sender:Content-Transfer-Encoding :Content-Type:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To:Message-Id:Date: Subject:Cc:To:From:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=Gme75XANzf3Ifobwt+8XeMi3Va2ZfIWEFHCFiFwk5v8=; b=VghvJilcQ45J62tVfzAgtXUy+ QPvSb0LQwYeuaIQZrYgwdpxiH0MPqYPZakbWbR58cyzwzO7Jpbd1Cv+dCgBQIUlrg0GIVFZyDcYXu kWLcc7cmD760cvKBuzmq7s8npHnf8odCbyiYzqlKpmQ0INgZHqj+inj0p52SRr95oKY4LO4wFNYoU v3+emeDtwCQxcOIrmI3Hk9htREaQu3HXVToX2D+vSntf42tK1nQymbk/iBUT8a54L60vmf95l53Li h9cs5VMLqLyhLCX9qQ/uQnhodUjds+Ok+HbwOcCt5UYE2/wpFVaPsDHJJBiSx49MMOnE3WUQS8aI1 djUaPfMKw==; Received: from localhost ([::1] helo=desiato.infradead.org) by desiato.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1liwgK-000ID1-QM; Tue, 18 May 2021 10:10:33 +0000 Received: from bombadil.infradead.org ([2607:7c80:54:e::133]) by desiato.infradead.org with esmtps (Exim 4.94 #2 (Red Hat Linux)) id 1liwLP-000E5G-Vi for linux-arm-kernel@desiato.infradead.org; Tue, 18 May 2021 09:48:56 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-Id:Date:Subject:Cc:To:From:Sender :Reply-To:Content-Type:Content-ID:Content-Description; bh=yANpBrPC8xtK64IcMuS6drLEs04eN8sfpSmdS9Ywib0=; b=Aj52/Q6jCWIFDoSCHm5SczokcQ C71c9y2WUzglV7n8ZTvG5NBoJ3gCjbc9AcByzzWzcWvXnYjhCqBZlQV4LSB+xFprcVzj2tIxsmt4I Kq9kKupm1n7UfBkm3WyQhfd8P55W0moxK6uUCZVYNdW9bxQEaaFLFOfVRryJpTOjYNlTzbhQ7NyYg de9ng4W87gBmXOMjNBquudOE+GaxJG2REXl9Lzuy/lXcQ6E61eSGM2//BCkBlDp9XvZl+HquUGPKH lkfwxygDb3EeDrrz/oEmdJj8LUKv8OhObF/M132YxSiKBjLebOMxKdBemmRh8lpo03Bw3uqPKtIam 9M1L5rWQ==; Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.94 #2 (Red Hat Linux)) id 1liwLM-00EWpC-Tw for linux-arm-kernel@lists.infradead.org; Tue, 18 May 2021 09:48:54 +0000 Received: by mail.kernel.org (Postfix) with ESMTPSA id 8011561404; Tue, 18 May 2021 09:48:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1621331332; bh=cLV1ruPvNZQWbLhrPTW5PTi59kS93zO0HxNZ8xc2n5I=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=PpUmRDuQPotlvWn6Zdm8S/5OhittrHyEVMvHp0uVcz4ZQfZit07yzmo/gvmt5fJsE cANOh+AlooAZYy7DCWLxMZulfgk9zCvL47X8Tn92OpSXCSPJeD3/tzsPsoTZXrUQkC vqHcsROicuUlVI0xVzlbqTLW7e+1kAKB8hVDeVnGS1ZX22G1ESF0XRx9ExjyuyR1OM gvoxgDN/VeMMK2hlyigf3JRWaHnKV60msIvhPv6RakToZXn5bVmQZpnB46ht3jbu8Z 5C6SSpXjuLXDH9n7Hwurn7z7rPVcR4gNK9M0rCfL/1BZha1gy5IxmVW1IvUiZCR5YY CpM3nGLV19GbQ== From: Will Deacon To: linux-arm-kernel@lists.infradead.org Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Will Deacon , Catalin Marinas , Marc Zyngier , Greg Kroah-Hartman , Peter Zijlstra , Morten Rasmussen , Qais Yousef , Suren Baghdasaryan , Quentin Perret , Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , "Rafael J. Wysocki" , kernel-team@android.com Subject: [PATCH v6 21/21] Documentation: arm64: describe asymmetric 32-bit support Date: Tue, 18 May 2021 10:47:25 +0100 Message-Id: <20210518094725.7701-22-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20210518094725.7701-1-will@kernel.org> References: <20210518094725.7701-1-will@kernel.org> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210518_024853_031118_694B2100 X-CRM114-Status: GOOD ( 32.31 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Document support for running 32-bit tasks on asymmetric 32-bit systems and its impact on the user ABI when enabled. Signed-off-by: Will Deacon --- .../admin-guide/kernel-parameters.txt | 3 + Documentation/arm64/asymmetric-32bit.rst | 149 ++++++++++++++++++ Documentation/arm64/index.rst | 1 + 3 files changed, 153 insertions(+) create mode 100644 Documentation/arm64/asymmetric-32bit.rst diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index a2e453919bb6..5a1dc7e628a5 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -295,6 +295,9 @@ EL0 is indicated by /sys/devices/system/cpu/aarch32_el0 and hot-unplug operations may be restricted. + See Documentation/arm64/asymmetric-32bit.rst for more + information. + amd_iommu= [HW,X86-64] Pass parameters to the AMD IOMMU driver in the system. Possible values are: diff --git a/Documentation/arm64/asymmetric-32bit.rst b/Documentation/arm64/asymmetric-32bit.rst new file mode 100644 index 000000000000..baf02c143363 --- /dev/null +++ b/Documentation/arm64/asymmetric-32bit.rst @@ -0,0 +1,149 @@ +====================== +Asymmetric 32-bit SoCs +====================== + +Author: Will Deacon + +This document describes the impact of asymmetric 32-bit SoCs on the +execution of 32-bit (``AArch32``) applications. + +Date: 2021-05-17 + +Introduction +============ + +Some Armv9 SoCs suffer from a big.LITTLE misfeature where only a subset +of the CPUs are capable of executing 32-bit user applications. On such +a system, Linux by default treats the asymmetry as a "mismatch" and +disables support for both the ``PER_LINUX32`` personality and +``execve(2)`` of 32-bit ELF binaries, with the latter returning +``-ENOEXEC``. If the mismatch is detected during late onlining of a +64-bit-only CPU, then the onlining operation fails and the new CPU is +unavailable for scheduling. + +Surprisingly, these SoCs have been produced with the intention of +running legacy 32-bit binaries. Unsurprisingly, that doesn't work very +well with the default behaviour of Linux. + +It seems inevitable that future SoCs will drop 32-bit support +altogether, so if you're stuck in the unenviable position of needing to +run 32-bit code on one of these transitionary platforms then you would +be wise to consider alternatives such as recompilation, emulation or +retirement. If neither of those options are practical, then read on. + +Enabling kernel support +======================= + +Since the kernel support is not completely transparent to userspace, +allowing 32-bit tasks to run on an asymmetric 32-bit system requires an +explicit "opt-in" and can be enabled by passing the +``allow_mismatched_32bit_el0`` parameter on the kernel command-line. + +For the remainder of this document we will refer to an *asymmetric +system* to mean an SoC running Linux with this kernel command-line +option enabled. + +Userspace impact +================ + +32-bit tasks running on an asymmetric system behave in mostly the same +way as on a homogeneous system, with a few key differences relating to +CPU affinity. + +sysfs +----- + +The subset of CPUs capable of running 32-bit tasks is described in +``/sys/devices/system/cpu/aarch32_el0`` and is documented further in +``Documentation/ABI/testing/sysfs-devices-system-cpu``. + +**Note:** CPUs are advertised by this file as they are detected and so +late-onlining of 32-bit-capable CPUs can result in the file contents +being modified by the kernel at runtime. Once advertised, CPUs are never +removed from the file. + +``execve(2)`` +------------- + +On a homogeneous system, the CPU affinity of a task is preserved across +``execve(2)``. This is not always possible on an asymmetric system, +specifically when the new program being executed is 32-bit yet the +affinity mask contains 64-bit-only CPUs. In this situation, the kernel +determines the new affinity mask as follows: + + 1. If the 32-bit-capable subset of the affinity mask is not empty, + then the affinity is restricted to that subset and the old affinity + mask is saved. This saved mask is inherited over ``fork(2)`` and + preserved across ``execve(2)`` of 32-bit programs. + + **Note:** This step does not apply to ``SCHED_DEADLINE`` tasks. + See `SCHED_DEADLINE`_. + + 2. Otherwise, the cpuset hierarchy of the task is walked until an + ancestor is found containing at least one 32-bit-capable CPU. The + affinity of the task is then changed to match the 32-bit-capable + subset of the cpuset determined by the walk. + + 3. On failure (i.e. out of memory), the affinity is changed to the set + of all 32-bit-capable CPUs of which the kernel is aware. + +A subsequent ``execve(2)`` of a 64-bit program by the 32-bit task will +invalidate the affinity mask saved in (1) and attempt to restore the CPU +affinity of the task using the saved mask if it was previously valid. +This restoration may fail due to intervening changes to the deadline +policy or cpuset hierarchy, in which case the ``execve(2)`` continues +with the affinity unchanged. + +Calls to ``sched_setaffinity(2)`` for a 32-bit task will consider only +the 32-bit-capable CPUs of the requested affinity mask. On success, the +affinity for the task is updated and any saved mask from a prior +``execve(2)`` is invalidated. + +``SCHED_DEADLINE`` +------------------ + +Admitting a 32-bit task to the deadline scheduler (e.g. by calling +``sched_setattr(2)``) will, if valid, consider the affinity mask saved +by a previous call to ``execve(2)`` for the purposes of input validation +in preference to the running affinity of the task. 64-bit deadline tasks +will skip step (1) of the process described in `execve(2)`_ when +executed a 32-bit program. + +**Note:** It is recommended that the 32-bit-capable CPUs are placed into +a separate root domain if ``SCHED_DEADLINE`` is to be used with 32-bit +tasks on an asymmetric system. Failure to do so is likely to result in +missed deadlines. + +Cpusets +------- + +The affinity of a 32-bit task may include CPUs that are not explicitly +allowed by the cpuset to which it is attached. This can occur as a +result of the following two situations: + + - A 64-bit task attached to a cpuset which allows only 64-bit CPUs + executes a 32-bit program. + + - All of the 32-bit-capable CPUs allowed by a cpuset containing a + 32-bit task are offlined. + +In both of these cases, the new affinity is calculated according to step +(2) of the process described in `execve(2)`_ and the cpuset hierarchy is +unchanged irrespective of the cgroup version. + +CPU hotplug +----------- + +When the kernel detects asymmetric 32-bit hardware, the first detected +32-bit-capable CPU is prevented from being offlined by userspace and any +such attempt will return ``-EPERM``. Note that suspend is still +permitted even if the primary CPU (i.e. CPU 0) is 64-bit-only. + +KVM +--- + +Although KVM will not advertise 32-bit EL0 support to any vCPUs on an +asymmetric system, a broken guest at EL1 could still attempt to execute +32-bit code at EL0. In this case, an exit from a vCPU thread in 32-bit +mode will return to host userspace with an ``exit_reason`` of +``KVM_EXIT_FAIL_ENTRY``. diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst index 97d65ba12a35..4f840bac083e 100644 --- a/Documentation/arm64/index.rst +++ b/Documentation/arm64/index.rst @@ -10,6 +10,7 @@ ARM64 Architecture acpi_object_usage amu arm-acpi + asymmetric-32bit booting cpu-feature-registers elf_hwcaps -- 2.31.1.751.gd2f1c929bd-goog _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel