From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Marc Zyngier <maz@kernel.org>,
Huacai Chen <chenhuacai@kernel.org>,
Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
Paul Mackerras <paulus@ozlabs.org>,
James Morse <james.morse@arm.com>,
Julien Thierry <julien.thierry.kdev@gmail.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org,
kvm@vger.kernel.org, kvm-ppc@vger.kernel.org,
linux-kernel@vger.kernel.org, Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
Date: Wed, 31 Mar 2021 22:30:05 +0200 [thread overview]
Message-ID: <fdd5bfb1-8abc-7658-b288-dc9943a6e04c@redhat.com> (raw)
In-Reply-To: <YGTYf9sWVIJqqswq@google.com>
On 31/03/21 22:15, Sean Christopherson wrote:
> On Wed, Mar 31, 2021, Paolo Bonzini wrote:
>> On 26/03/21 03:19, Sean Christopherson wrote:
>>> + /*
>>> + * Reset the lock used to prevent memslot updates between MMU notifier
>>> + * range_start and range_end. At this point no more MMU notifiers will
>>> + * run, but the lock could still be held if KVM's notifier was removed
>>> + * between range_start and range_end. No threads can be waiting on the
>>> + * lock as the last reference on KVM has been dropped. If the lock is
>>> + * still held, freeing memslots will deadlock.
>>> + */
>>> + init_rwsem(&kvm->mmu_notifier_slots_lock);
>>
>> I was going to say that this is nasty, then I noticed that
>> mmu_notifier_unregister uses SRCU to ensure completion of concurrent calls
>> to the MMU notifier. So I guess it's fine, but it's better to point it out:
>>
>> /*
>> * At this point no more MMU notifiers will run and pending
>> * calls to range_start have completed, but the lock would
>> * still be held and never released if the MMU notifier was
>> * removed between range_start and range_end. Since the last
>> * reference to the struct kvm has been dropped, no threads can
>> * be waiting on the lock, but we might still end up taking it
>> * when freeing memslots in kvm_arch_destroy_vm. Reset the lock
>> * to avoid deadlocks.
>> */
>
> An alternative would be to not take the lock in install_new_memslots() if
> kvm->users_count == 0. It'd be weirder to document, and the conditional locking
> would still be quite ugly. Not sure if that's better than blasting a lock
> during destruction?
No, that's worse...
Paolo
next prev parent reply other threads:[~2021-03-31 20:31 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-26 2:19 [PATCH 00/18] KVM: Consolidate and optimize MMU notifiers Sean Christopherson
2021-03-26 2:19 ` [PATCH 01/18] KVM: x86/mmu: Coalesce TDP MMU TLB flushes when zapping collapsible SPTEs Sean Christopherson
2021-03-26 2:19 ` [PATCH 02/18] KVM: x86/mmu: Move flushing for "slot" handlers to caller for legacy MMU Sean Christopherson
2021-03-26 2:19 ` [PATCH 03/18] KVM: x86/mmu: Coalesce TLB flushes when zapping collapsible SPTEs Sean Christopherson
2021-03-26 2:19 ` [PATCH 04/18] KVM: x86/mmu: Coalesce TLB flushes across address spaces for gfn range zap Sean Christopherson
2021-03-26 2:19 ` [PATCH 05/18] KVM: x86/mmu: Pass address space ID to __kvm_tdp_mmu_zap_gfn_range() Sean Christopherson
2021-03-26 2:19 ` [PATCH 06/18] KVM: x86/mmu: Pass address space ID to TDP MMU root walkers Sean Christopherson
2021-03-26 2:19 ` [PATCH 07/18] KVM: x86/mmu: Use leaf-only loop for walking TDP SPTEs when changing SPTE Sean Christopherson
2021-03-26 2:19 ` [PATCH 08/18] KVM: Move prototypes for MMU notifier callbacks to generic code Sean Christopherson
2021-03-26 2:19 ` [PATCH 09/18] KVM: Move arm64's MMU notifier trace events " Sean Christopherson
2021-03-26 2:19 ` [PATCH 10/18] KVM: Move x86's MMU notifier memslot walkers " Sean Christopherson
2021-03-31 7:52 ` Paolo Bonzini
2021-03-31 16:20 ` Sean Christopherson
2021-03-31 16:36 ` Paolo Bonzini
2021-03-26 2:19 ` [PATCH 11/18] KVM: arm64: Convert to the gfn-based MMU notifier callbacks Sean Christopherson
2021-03-26 2:19 ` [PATCH 12/18] KVM: MIPS/MMU: " Sean Christopherson
2021-03-31 7:41 ` Paolo Bonzini
2021-03-26 2:19 ` [PATCH 13/18] KVM: PPC: " Sean Christopherson
2021-03-26 2:19 ` [PATCH 14/18] KVM: Kill off the old hva-based " Sean Christopherson
2021-03-26 2:19 ` [PATCH 15/18] KVM: Take mmu_lock when handling MMU notifier iff the hva hits a memslot Sean Christopherson
2021-03-26 2:19 ` [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary Sean Christopherson
2021-03-31 7:52 ` Paolo Bonzini
2021-03-31 8:35 ` Paolo Bonzini
2021-03-31 16:41 ` Sean Christopherson
2021-03-31 16:47 ` Paolo Bonzini
2021-03-31 19:47 ` Sean Christopherson
2021-03-31 20:42 ` Paolo Bonzini
2021-03-31 21:05 ` Sean Christopherson
2021-03-31 21:22 ` Sean Christopherson
2021-03-31 21:36 ` Paolo Bonzini
2021-03-31 21:35 ` Paolo Bonzini
2021-03-31 21:47 ` Sean Christopherson
2021-03-31 20:15 ` Sean Christopherson
2021-03-31 20:30 ` Paolo Bonzini [this message]
2021-03-31 20:52 ` Sean Christopherson
2021-03-31 21:00 ` Paolo Bonzini
2021-03-26 2:19 ` [PATCH 17/18] KVM: x86/mmu: Allow yielding during MMU notifier unmap/zap, if possible Sean Christopherson
2021-03-26 2:19 ` [PATCH 18/18] KVM: x86/mmu: Drop trace_kvm_age_page() tracepoint Sean Christopherson
2021-03-30 18:32 ` [PATCH 00/18] KVM: Consolidate and optimize MMU notifiers Ben Gardon
2021-03-30 19:48 ` Paolo Bonzini
2021-03-30 19:58 ` Sean Christopherson
2021-03-31 7:57 ` Paolo Bonzini
2021-03-31 9:34 ` Marc Zyngier
2021-03-31 9:41 ` Paolo Bonzini
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