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From: Sean Christopherson <seanjc@google.com>
To: Marc Zyngier <maz@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
	Paul Mackerras <paulus@ozlabs.org>,
	Paolo Bonzini <pbonzini@redhat.com>
Cc: James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Sean Christopherson <seanjc@google.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: [PATCH v2 01/10] KVM: Assert that notifier count is elevated in .change_pte()
Date: Thu,  1 Apr 2021 17:56:49 -0700
Message-ID: <20210402005658.3024832-2-seanjc@google.com> (raw)
In-Reply-To: <20210402005658.3024832-1-seanjc@google.com>

In KVM's .change_pte() notification callback, replace the notifier
sequence bump with a WARN_ON assertion that the notifier count is
elevated.  An elevated count provides stricter protections than bumping
the sequence, and the sequence is guarnateed to be bumped before the
count hits zero.

When .change_pte() was added by commit 828502d30073 ("ksm: add
mmu_notifier set_pte_at_notify()"), bumping the sequence was necessary
as .change_pte() would be invoked without any surrounding notifications.

However, since commit 6bdb913f0a70 ("mm: wrap calls to set_pte_at_notify
with invalidate_range_start and invalidate_range_end"), all calls to
.change_pte() are guaranteed to be bookended by start() and end(), and
so are guaranteed to run with an elevated notifier count.

Note, wrapping .change_pte() with .invalidate_range_{start,end}() is a
bug of sorts, as invalidating the secondary MMU's (KVM's) PTE defeats
the purpose of .change_pte().  Every arch's kvm_set_spte_hva() assumes
.change_pte() is called when the relevant SPTE is present in KVM's MMU,
as the original goal was to accelerate Kernel Samepage Merging (KSM) by
updating KVM's SPTEs without requiring a VM-Exit (due to invalidating
the SPTE).  I.e. it means that .change_pte() is effectively dead code
on _all_ architectures.

x86 and MIPS are clearcut nops if the old SPTE is not-present, and that
is guaranteed due to the prior invalidation.  PPC simply unmaps the SPTE,
which again should be a nop due to the invalidation.  arm64 is a bit
murky, but it's also likely a nop because kvm_pgtable_stage2_map() is
called without a cache pointer, which means it will map an entry if and
only if an existing PTE was found.

For now, take advantage of the bug to simplify future consolidation of
KVMs's MMU notifier code.   Doing so will not greatly complicate fixing
.change_pte(), assuming it's even worth fixing.  .change_pte() has been
broken for 8+ years and no one has complained.  Even if there are
KSM+KVM users that care deeply about its performance, the benefits of
avoiding VM-Exits via .change_pte() need to be reevaluated to justify
the added complexity and testing burden.  Ripping out .change_pte()
entirely would be a lot easier.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d1de843b7618..8df091950161 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -461,12 +461,17 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
 
 	trace_kvm_set_spte_hva(address);
 
+	/*
+	 * .change_pte() must be bookended by .invalidate_range_{start,end}(),
+	 * and so always runs with an elevated notifier count.  This obviates
+	 * the need to bump the sequence count.
+	 */
+	WARN_ON_ONCE(!kvm->mmu_notifier_count);
+
 	idx = srcu_read_lock(&kvm->srcu);
 
 	KVM_MMU_LOCK(kvm);
 
-	kvm->mmu_notifier_seq++;
-
 	if (kvm_set_spte_hva(kvm, address, pte))
 		kvm_flush_remote_tlbs(kvm);
 
-- 
2.31.0.208.g409f899ff0-goog


  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 ` Sean Christopherson [this message]
2021-04-02 11:08   ` [PATCH v2 01/10] KVM: Assert that notifier count is elevated in .change_pte() 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
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

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