From patchwork Thu Jun 10 12:06:14 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Paolo Bonzini X-Patchwork-Id: 12312963 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=-15.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, 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 8C79FC48BE0 for ; Thu, 10 Jun 2021 12:06:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 792A4613E7 for ; Thu, 10 Jun 2021 12:06:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230370AbhFJMIY (ORCPT ); Thu, 10 Jun 2021 08:08:24 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:49453 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230343AbhFJMIT (ORCPT ); Thu, 10 Jun 2021 08:08:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1623326783; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Jl9kawmuXJHQDXlliaeVf+5NhkAChMTRODp32/eFII4=; b=XKFZLYLCGqDGjR6mcW+l1wQXS6eaLAaSdORSeijJGhj00Pg8WdPyqughW5NZQmX2t2WBRd drygHaz2VB/z8YBh3HwDcrs82jDnjaUacB/1IM8GUvfqOPP1uu2gFEpKlk6Zdp24p3TL+Q 73eX6gBiVC/Lb8TPWDeQAgcVCbupX0U= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-46-Yk0pAoa5Oq-amFcH9KDXDw-1; Thu, 10 Jun 2021 08:06:18 -0400 X-MC-Unique: Yk0pAoa5Oq-amFcH9KDXDw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 766D1C73A0; Thu, 10 Jun 2021 12:06:17 +0000 (UTC) Received: from virtlab701.virt.lab.eng.bos.redhat.com (virtlab701.virt.lab.eng.bos.redhat.com [10.19.152.228]) by smtp.corp.redhat.com (Postfix) with ESMTP id 158EE19D7C; Thu, 10 Jun 2021 12:06:17 +0000 (UTC) From: Paolo Bonzini To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: seanjc@google.com, bgardon@google.com Subject: [PATCH 1/2] KVM: Block memslot updates across range_start() and range_end() Date: Thu, 10 Jun 2021 08:06:14 -0400 Message-Id: <20210610120615.172224-2-pbonzini@redhat.com> In-Reply-To: <20210610120615.172224-1-pbonzini@redhat.com> References: <20210610120615.172224-1-pbonzini@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org We would like to avoid taking mmu_lock for .invalidate_range_{start,end}() notifications that are unrelated to KVM. Because mmu_notifier_count must be modified while holding mmu_lock for write, and must always be paired across start->end to stay balanced, lock elision must happen in both or none. Therefore, in preparation for this change, this patch prevents memslot updates across range_start() and range_end(). Note, technically flag-only memslot updates could be allowed in parallel, but stalling a memslot update for a relatively short amount of time is not a scalability issue, and this is all more than complex enough. A long note on the locking: a previous version of the patch used an rwsem to block the memslot update while the MMU notifier run, but this resulted in the following deadlock involving the pseudo-lock tagged as "mmu_notifier_invalidate_range_start". ====================================================== WARNING: possible circular locking dependency detected 5.12.0-rc3+ #6 Tainted: G OE ------------------------------------------------------ qemu-system-x86/3069 is trying to acquire lock: ffffffff9c775ca0 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}, at: __mmu_notifier_invalidate_range_end+0x5/0x190 but task is already holding lock: ffffaff7410a9160 (&kvm->mmu_notifier_slots_lock){.+.+}-{3:3}, at: kvm_mmu_notifier_invalidate_range_start+0x36d/0x4f0 [kvm] which lock already depends on the new lock. This corresponds to the following MMU notifier logic: invalidate_range_start take pseudo lock down_read() (*) release pseudo lock invalidate_range_end take pseudo lock (**) up_read() release pseudo lock At point (*) we take the mmu_notifiers_slots_lock inside the pseudo lock; at point (**) we take the pseudo lock inside the mmu_notifiers_slots_lock. This could cause a deadlock (ignoring for a second that the pseudo lock is not a lock): - invalidate_range_start waits on down_read(), because the rwsem is held by install_new_memslots - install_new_memslots waits on down_write(), because the rwsem is held till (another) invalidate_range_end finishes - invalidate_range_end sits waits on the pseudo lock, held by invalidate_range_start. Removing the fairness of the rwsem breaks the cycle (in lockdep terms, it would change the *shared* rwsem readers into *shared recursive* readers), so open-code the wait using a readers count and a spinlock. This also allows handling blockable and non-blockable critical section in the same way. Losing the rwsem fairness does theoretically allow MMU notifiers to block install_new_memslots forever. Note that mm/mmu_notifier.c's own retry scheme in mmu_interval_read_begin also uses wait/wake_up and is likewise not fair. Signed-off-by: Paolo Bonzini --- Documentation/virt/kvm/locking.rst | 6 +++ include/linux/kvm_host.h | 10 ++++- virt/kvm/kvm_main.c | 61 ++++++++++++++++++++++++++++-- 3 files changed, 72 insertions(+), 5 deletions(-) diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst index 35eca377543d..8138201efb09 100644 --- a/Documentation/virt/kvm/locking.rst +++ b/Documentation/virt/kvm/locking.rst @@ -21,6 +21,12 @@ The acquisition orders for mutexes are as follows: can be taken inside a kvm->srcu read-side critical section, while kvm->slots_lock cannot. +- kvm->mn_active_invalidate_count ensures that pairs of + invalidate_range_start() and invalidate_range_end() callbacks + use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock + are taken on the waiting side in install_new_memslots, so MMU notifiers + must not take either kvm->slots_lock or kvm->slots_arch_lock. + On x86: - vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 11b9b11a5e9b..82bd62cf45d3 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -536,6 +536,11 @@ struct kvm { struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM]; struct kvm_vcpu *vcpus[KVM_MAX_VCPUS]; + /* Used to wait for completion of MMU notifiers. */ + spinlock_t mn_invalidate_lock; + unsigned long mn_active_invalidate_count; + struct rcuwait mn_memslots_update_rcuwait; + /* * created_vcpus is protected by kvm->lock, and is incremented * at the beginning of KVM_CREATE_VCPU. online_vcpus is only @@ -721,8 +726,9 @@ static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id) { as_id = array_index_nospec(as_id, KVM_ADDRESS_SPACE_NUM); return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu, - lockdep_is_held(&kvm->slots_lock) || - !refcount_read(&kvm->users_count)); + lockdep_is_held(&kvm->slots_lock) || + READ_ONCE(kvm->mn_active_invalidate_count) || + !refcount_read(&kvm->users_count)); } static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index fa7e7ebefc79..0dc0726c8d18 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -605,10 +605,13 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn, /* * .change_pte() must be surrounded by .invalidate_range_{start,end}(), - * and so always runs with an elevated notifier count. This obviates - * the need to bump the sequence count. + * If mmu_notifier_count is zero, then start() didn't find a relevant + * memslot and wasn't forced down the slow path; rechecking here is + * unnecessary. */ - WARN_ON_ONCE(!kvm->mmu_notifier_count); + WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count)); + if (!kvm->mmu_notifier_count) + return; kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn); } @@ -658,6 +661,18 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn, trace_kvm_unmap_hva_range(range->start, range->end); + /* + * Prevent memslot modification between range_start() and range_end() + * so that conditionally locking provides the same result in both + * functions. Without that guarantee, the mmu_notifier_count + * adjustments will be imbalanced. + * + * Pairs with the decrement in range_end(). + */ + spin_lock(&kvm->mn_invalidate_lock); + kvm->mn_active_invalidate_count++; + spin_unlock(&kvm->mn_invalidate_lock); + __kvm_handle_hva_range(kvm, &hva_range); return 0; @@ -694,9 +709,22 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn, .flush_on_ret = false, .may_block = mmu_notifier_range_blockable(range), }; + bool wake; __kvm_handle_hva_range(kvm, &hva_range); + /* Pairs with the increment in range_start(). */ + spin_lock(&kvm->mn_invalidate_lock); + wake = (--kvm->mn_active_invalidate_count == 0); + spin_unlock(&kvm->mn_invalidate_lock); + + /* + * There can only be one waiter, since the wait happens under + * slots_lock. + */ + if (wake) + rcuwait_wake_up(&kvm->mn_memslots_update_rcuwait); + BUG_ON(kvm->mmu_notifier_count < 0); } @@ -910,6 +938,9 @@ static struct kvm *kvm_create_vm(unsigned long type) mutex_init(&kvm->irq_lock); mutex_init(&kvm->slots_lock); mutex_init(&kvm->slots_arch_lock); + spin_lock_init(&kvm->mn_invalidate_lock); + rcuwait_init(&kvm->mn_memslots_update_rcuwait); + INIT_LIST_HEAD(&kvm->devices); BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX); @@ -1030,6 +1061,16 @@ static void kvm_destroy_vm(struct kvm *kvm) kvm_coalesced_mmio_free(kvm); #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER) mmu_notifier_unregister(&kvm->mmu_notifier, kvm->mm); + /* + * At this point, pending calls to invalidate_range_start() + * have completed but no more MMU notifiers will run, so + * mn_active_invalidate_count may remain unbalanced. + * No threads can be waiting in install_new_memslots as the + * last reference on KVM has been dropped, but freeing + * memslots would deadlock without this manual intervention. + */ + WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait)); + kvm->mn_active_invalidate_count = 0; #else kvm_arch_flush_shadow_all(kvm); #endif @@ -1281,7 +1322,21 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, WARN_ON(gen & KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS); slots->generation = gen | KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS; + /* + * Do not store the new memslots while there are invalidations in + * progress (preparatory change for the next commit). + */ + spin_lock(&kvm->mn_invalidate_lock); + prepare_to_rcuwait(&kvm->mn_memslots_update_rcuwait); + while (kvm->mn_active_invalidate_count) { + set_current_state(TASK_UNINTERRUPTIBLE); + spin_unlock(&kvm->mn_invalidate_lock); + schedule(); + spin_lock(&kvm->mn_invalidate_lock); + } + finish_rcuwait(&kvm->mn_memslots_update_rcuwait); rcu_assign_pointer(kvm->memslots[as_id], slots); + spin_unlock(&kvm->mn_invalidate_lock); /* * Acquired in kvm_set_memslot. Must be released before synchronize