From patchwork Wed Nov 30 23:09:28 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sean Christopherson X-Patchwork-Id: 13060631 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E03BBC47089 for ; Wed, 30 Nov 2022 23:16:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230183AbiK3XQl (ORCPT ); Wed, 30 Nov 2022 18:16:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58660 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230184AbiK3XQI (ORCPT ); Wed, 30 Nov 2022 18:16:08 -0500 Received: from mail-pg1-x549.google.com (mail-pg1-x549.google.com [IPv6:2607:f8b0:4864:20::549]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E6EB4A323A for ; Wed, 30 Nov 2022 15:11:35 -0800 (PST) Received: by mail-pg1-x549.google.com with SMTP id 69-20020a630148000000b00478118684c4so56699pgb.20 for ; Wed, 30 Nov 2022 15:11:35 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:from:to:cc:subject:date:message-id:reply-to; bh=pe1Kkjvkgy8PDlwU67QIc0+aecYuhy5E1AiDnDSceyU=; b=PtfTeuxltMAWC2r28YkEpC42/mHVqlVK9cbH6+HjFFnh8l3fnLdSvP/QyQcZjiH0fJ bAAP0a8yCSDYKaUdaH21SBJdNohuMejKBiChzsMvrmU4O6PwxgsyB11gpo8HZ1qpOnNj 0KjvQaXNCIfI/gT8JFhhVnnRkflSvNYyY4jp8svnC7Lan9kSJyrM/U9WEp+pXRwL/qXE QPH/u84NCMboZU39GSXi9Lj4fcmoi1xI+xbEkS68VCcV+znlZLd7AdaPCKmQ9tBL0Okt aG5KOLnbOknW0mumwkbW75rjZaylsDPJeJ0+up/D0vzJ/h6wHyc7pmAOmpfzYxQCQRg1 XCJA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=pe1Kkjvkgy8PDlwU67QIc0+aecYuhy5E1AiDnDSceyU=; b=kz7tKEcmozHx3JcjjeAYhlUW/Myb1RELauLCyfh3vBa/VhwV+QNsduIW1I9GTdl7fM ebwggk2RCMMmxI46DNvANBIX/oJhK0qKScdcEHlsqtmHoDVSZhp8On5mLA4LIlwkgCca ibNSlygc0Li7FQcUay9Riu55K8WQLiyq1JFUeYHImuVNtUnw3wG/u3s7xDSnj8Cn590p 7LBcCf0HDGaeSrgIwbEjmcfoZ8GrdHRE9UVeREcNvrNRzgRAz4BMsPjmBJyGW9Y73IeE Dhv+b3Ygh0YaN+iCt7vM56tMwFvpvWeOEFiqNYdyolLGiCD/dAeGI27bEWLRlvYnIaUs 6UGA== X-Gm-Message-State: ANoB5pnOO6B2N+oTCHkhjpxgN6/xS3M7K71BepPXYMxXvH7moyofqHSV nDshVKScjr+wQdnyt44UcMNflJNwvbE= X-Google-Smtp-Source: AA0mqf4jOwpuCSGbn98bo3vks+P0Y1utEHPDSunPwfv+lom3uyEWpWcpvAjoSiednnXdmVtMG3ik4xfUbWU= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:6a00:2c87:b0:562:e790:dfe0 with SMTP id ef7-20020a056a002c8700b00562e790dfe0mr65114033pfb.16.1669849853870; Wed, 30 Nov 2022 15:10:53 -0800 (PST) Reply-To: Sean Christopherson Date: Wed, 30 Nov 2022 23:09:28 +0000 In-Reply-To: <20221130230934.1014142-1-seanjc@google.com> Mime-Version: 1.0 References: <20221130230934.1014142-1-seanjc@google.com> X-Mailer: git-send-email 2.38.1.584.g0f3c55d4c2-goog Message-ID: <20221130230934.1014142-45-seanjc@google.com> Subject: [PATCH v2 44/50] KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock From: Sean Christopherson To: Paolo Bonzini , Marc Zyngier , Huacai Chen , Aleksandar Markovic , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Matthew Rosato , Eric Farman , Sean Christopherson , Vitaly Kuznetsov , David Woodhouse , Paul Durrant Cc: James Morse , Alexandru Elisei , Suzuki K Poulose , Oliver Upton , Atish Patra , David Hildenbrand , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, Yuan Yao , Cornelia Huck , Isaku Yamahata , " =?utf-8?q?Philippe_Mathieu-Daud=C3=A9?= " , Fabiano Rosas , Michael Ellerman , Kai Huang , Chao Gao , Thomas Gleixner Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org From: Isaku Yamahata Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock now that KVM hooks CPU hotplug during the ONLINE phase, which can sleep. Previously, KVM hooked the STARTING phase, which is not allowed to sleep and thus could not take kvm_lock (a mutex). This effectively allows the task that's initiating hardware enabling/disabling to preempted and/or migrated. Note, the Documentation/virt/kvm/locking.rst statement that kvm_count_lock is "raw" because hardware enabling/disabling needs to be atomic with respect to migration is wrong on multiple fronts. First, while regular spinlocks can be preempted, the task holding the lock cannot be migrated. Second, preventing migration is not required. on_each_cpu() disables preemption, which ensures that cpus_hardware_enabled correctly reflects hardware state. The task may be preempted/migrated between bumping kvm_usage_count and invoking on_each_cpu(), but that's perfectly ok as kvm_usage_count is still protected, e.g. other tasks that call hardware_enable_all() will be blocked until the preempted/migrated owner exits its critical section. KVM does have lockless accesses to kvm_usage_count in the suspend/resume flows, but those are safe because all tasks must be frozen prior to suspending CPUs, and a task cannot be frozen while it holds one or more locks (userspace tasks are frozen via a fake signal). Preemption doesn't need to be explicitly disabled in the hotplug path. The hotplug thread is pinned to the CPU that's being hotplugged, and KVM only cares about having a stable CPU, i.e. to ensure hardware is enabled on the correct CPU. Lockep, i.e. check_preemption_disabled(), plays nice with this state too, as is_percpu_thread() is true for the hotplug thread. Signed-off-by: Isaku Yamahata Co-developed-by: Sean Christopherson Signed-off-by: Sean Christopherson --- Documentation/virt/kvm/locking.rst | 19 ++++++++-------- virt/kvm/kvm_main.c | 36 ++++++++++++++++++++---------- 2 files changed, 34 insertions(+), 21 deletions(-) diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst index 132a9e5436e5..cd570e565522 100644 --- a/Documentation/virt/kvm/locking.rst +++ b/Documentation/virt/kvm/locking.rst @@ -9,6 +9,8 @@ KVM Lock Overview The acquisition orders for mutexes are as follows: +- cpus_read_lock() is taken outside kvm_lock + - kvm->lock is taken outside vcpu->mutex - kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock @@ -216,15 +218,10 @@ time it will be set using the Dirty tracking mechanism described above. :Type: mutex :Arch: any :Protects: - vm_list - -``kvm_count_lock`` -^^^^^^^^^^^^^^^^^^ - -:Type: raw_spinlock_t -:Arch: any -:Protects: - hardware virtualization enable/disable -:Comment: 'raw' because hardware enabling/disabling must be atomic /wrt - migration. + - kvm_usage_count + - hardware virtualization enable/disable +:Comment: KVM also disables CPU hotplug via cpus_read_lock() during + enable/disable. ``kvm->mn_invalidate_lock`` ^^^^^^^^^^^^^^^^^^^^^^^^^^^ @@ -288,3 +285,7 @@ time it will be set using the Dirty tracking mechanism described above. :Type: mutex :Arch: x86 :Protects: loading a vendor module (kvm_amd or kvm_intel) +:Comment: Exists because using kvm_lock leads to deadlock. cpu_hotplug_lock is + taken outside of kvm_lock, e.g. in KVM's CPU online/offline callbacks, and + many operations need to take cpu_hotplug_lock when loading a vendor module, + e.g. updating static calls. diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index a46d61e9c053..6a8fb53b32f0 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -100,7 +100,6 @@ EXPORT_SYMBOL_GPL(halt_poll_ns_shrink); */ DEFINE_MUTEX(kvm_lock); -static DEFINE_RAW_SPINLOCK(kvm_count_lock); LIST_HEAD(vm_list); static cpumask_var_t cpus_hardware_enabled; @@ -5054,17 +5053,18 @@ static int kvm_online_cpu(unsigned int cpu) * be enabled. Otherwise running VMs would encounter unrecoverable * errors when scheduled to this CPU. */ - raw_spin_lock(&kvm_count_lock); + mutex_lock(&kvm_lock); if (kvm_usage_count) { WARN_ON_ONCE(atomic_read(&hardware_enable_failed)); hardware_enable_nolock(NULL); + if (atomic_read(&hardware_enable_failed)) { atomic_set(&hardware_enable_failed, 0); ret = -EIO; } } - raw_spin_unlock(&kvm_count_lock); + mutex_unlock(&kvm_lock); return ret; } @@ -5080,10 +5080,10 @@ static void hardware_disable_nolock(void *junk) static int kvm_offline_cpu(unsigned int cpu) { - raw_spin_lock(&kvm_count_lock); + mutex_lock(&kvm_lock); if (kvm_usage_count) hardware_disable_nolock(NULL); - raw_spin_unlock(&kvm_count_lock); + mutex_unlock(&kvm_lock); return 0; } @@ -5099,9 +5099,9 @@ static void hardware_disable_all_nolock(void) static void hardware_disable_all(void) { cpus_read_lock(); - raw_spin_lock(&kvm_count_lock); + mutex_lock(&kvm_lock); hardware_disable_all_nolock(); - raw_spin_unlock(&kvm_count_lock); + mutex_unlock(&kvm_lock); cpus_read_unlock(); } @@ -5118,7 +5118,7 @@ static int hardware_enable_all(void) * enable hardware multiple times. */ cpus_read_lock(); - raw_spin_lock(&kvm_count_lock); + mutex_lock(&kvm_lock); kvm_usage_count++; if (kvm_usage_count == 1) { @@ -5131,7 +5131,7 @@ static int hardware_enable_all(void) } } - raw_spin_unlock(&kvm_count_lock); + mutex_unlock(&kvm_lock); cpus_read_unlock(); return r; @@ -5737,6 +5737,17 @@ static void kvm_init_debug(void) static int kvm_suspend(void) { + /* + * Secondary CPUs and CPU hotplug are disabled across the suspend/resume + * callbacks, i.e. no need to acquire kvm_lock to ensure the usage count + * is stable. Assert that kvm_lock is not held to ensure the system + * isn't suspended while KVM is enabling hardware. Hardware enabling + * can be preempted, but the task cannot be frozen until it has dropped + * all locks (userspace tasks are frozen via a fake signal). + */ + lockdep_assert_not_held(&kvm_lock); + lockdep_assert_irqs_disabled(); + if (kvm_usage_count) hardware_disable_nolock(NULL); return 0; @@ -5744,10 +5755,11 @@ static int kvm_suspend(void) static void kvm_resume(void) { - if (kvm_usage_count) { - lockdep_assert_not_held(&kvm_count_lock); + lockdep_assert_not_held(&kvm_lock); + lockdep_assert_irqs_disabled(); + + if (kvm_usage_count) hardware_enable_nolock(NULL); - } } static struct syscore_ops kvm_syscore_ops = {