KVM Archive on lore.kernel.org
 help / color / Atom feed
From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
	Paul Mackerras <paulus@ozlabs.org>
Cc: 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 v2 09/10] KVM: Don't take mmu_lock for range invalidation unless necessary
Date: Fri, 2 Apr 2021 11:34:38 +0200
Message-ID: <417bd6b5-b7d0-ed22-adae-02150cdbfebe@redhat.com> (raw)
In-Reply-To: <20210402005658.3024832-10-seanjc@google.com>

On 02/04/21 02:56, Sean Christopherson wrote:
> Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}()
> notifications.  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.  To meet that
> requirement, add a rwsem to prevent memslot updates across range_start()
> and range_end().
> 
> Use a rwsem instead of a rwlock since most notifiers _allow_ blocking,
> and the lock will be endl across the entire start() ... end() sequence.
> If anything in the sequence sleeps, including the caller or a different
> notifier, holding the spinlock would be disastrous.
> 
> For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down
> the slow path of unconditionally acquiring mmu_lock.  The sane
> alternative would be to try to acquire the lock and force the notifier
> to retry on failure.  But since OOM is currently the _only_ scenario
> where blocking is disallowed attempting to optimize a guest that has been
> marked for death is pointless.
> 
> Unconditionally define and use mmu_notifier_slots_lock in the memslots
> code, purely to avoid more #ifdefs.  The overhead of acquiring the lock
> is negligible when the lock is uncontested, which will always be the case
> when the MMU notifiers are not used.
> 
> 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.

Proposal for the locking documentation:

diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index b21a34c34a21..3e4ad7de36cb 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -16,6 +16,13 @@ The acquisition orders for mutexes are as follows:
  - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
    them together is quite rare.
  
+- The kvm->mmu_notifier_slots_lock rwsem ensures that pairs of
+  invalidate_range_start() and invalidate_range_end() callbacks
+  use the same memslots array.  kvm->slots_lock is taken outside the
+  write-side critical section of kvm->mmu_notifier_slots_lock, so
+  MMU notifiers must not take kvm->slots_lock.  No other write-side
+  critical sections should be added.
+
  On x86, vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock.
  
  Everything else is a leaf: no other lock is taken inside the critical

Paolo


  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-02  0:56 [PATCH v2 00/10] KVM: Consolidate and optimize MMU notifiers Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 01/10] KVM: Assert that notifier count is elevated in .change_pte() Sean Christopherson
2021-04-02 11:08   ` Paolo Bonzini
2021-04-02  0:56 ` [PATCH v2 02/10] KVM: Move x86's MMU notifier memslot walkers to generic code Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 03/10] KVM: arm64: Convert to the gfn-based MMU notifier callbacks Sean Christopherson
2021-04-12 10:12   ` Marc Zyngier
2021-04-02  0:56 ` [PATCH v2 04/10] KVM: MIPS/MMU: " Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 05/10] KVM: PPC: " Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 06/10] KVM: Kill off the old hva-based " Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 07/10] KVM: Move MMU notifier's mmu_lock acquisition into common helper Sean Christopherson
2021-04-02  9:35   ` Paolo Bonzini
2021-04-02 14:59     ` Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 08/10] KVM: Take mmu_lock when handling MMU notifier iff the hva hits a memslot Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 09/10] KVM: Don't take mmu_lock for range invalidation unless necessary Sean Christopherson
2021-04-02  9:34   ` Paolo Bonzini [this message]
2021-04-02 14:59     ` Sean Christopherson
2021-04-19  8:49   ` Wanpeng Li
2021-04-19 13:50     ` Paolo Bonzini
2021-04-19 15:09       ` Sean Christopherson
2021-04-19 22:09         ` Paolo Bonzini
2021-04-20  1:17           ` Sean Christopherson
2021-04-02  0:56 ` [PATCH v2 10/10] KVM: x86/mmu: Allow yielding during MMU notifier unmap/zap, if possible Sean Christopherson
2021-04-02 12:17 ` [PATCH v2 00/10] KVM: Consolidate and optimize MMU notifiers Paolo Bonzini
2021-04-12 10:27   ` Marc Zyngier

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=417bd6b5-b7d0-ed22-adae-02150cdbfebe@redhat.com \
    --to=pbonzini@redhat.com \
    --cc=aleksandar.qemu.devel@gmail.com \
    --cc=bgardon@google.com \
    --cc=chenhuacai@kernel.org \
    --cc=james.morse@arm.com \
    --cc=jmattson@google.com \
    --cc=joro@8bytes.org \
    --cc=julien.thierry.kdev@gmail.com \
    --cc=kvm-ppc@vger.kernel.org \
    --cc=kvm@vger.kernel.org \
    --cc=kvmarm@lists.cs.columbia.edu \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mips@vger.kernel.org \
    --cc=maz@kernel.org \
    --cc=paulus@ozlabs.org \
    --cc=seanjc@google.com \
    --cc=suzuki.poulose@arm.com \
    --cc=vkuznets@redhat.com \
    --cc=wanpengli@tencent.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

KVM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/kvm/0 kvm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kvm kvm/ https://lore.kernel.org/kvm \
		kvm@vger.kernel.org
	public-inbox-index kvm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.kvm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git