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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 10/18] KVM: Move x86's MMU notifier memslot walkers to generic code
Date: Wed, 31 Mar 2021 18:36:44 +0200	[thread overview]
Message-ID: <d843bbd5-d3f0-3cda-aaa5-8e0fb1d69554@redhat.com> (raw)
In-Reply-To: <YGShRP9E49p3vcos@google.com>

On 31/03/21 18:20, Sean Christopherson wrote:
> Every call
> to .change_pte() is bookended by .invalidate_range_{start,end}(), i.e. the above
> missing kvm->mmu_notifier_seq++ is benign because kvm->mmu_notifier_count is
> guaranteed to be non-zero.

In fact ARM even relies on invalidate wrapping the change_pte handler.

         /*
          * The MMU notifiers will have unmapped a huge PMD before calling
          * ->change_pte() (which in turn calls kvm_set_spte_hva()) and
          * therefore we never need to clear out a huge PMD through this
          * calling path and a memcache is not required.
          */

> Assuming all of the above is correct, I'm very tempted to rip out .change_pte()
> entirely.

There is still the performance benefit from immediately remapping the 
page to the new destination without waiting for a fault.  Yes it's 
hypothetical but I would prefer to leave that change for later.

The fact that the count is nonzero means that you will not even have to 
complicate kvm_mmu_notifier_change_pte to handle the removal of 
mmu_notifier_seq; just add a patch before this one to WARN if it is 
zero.  (The rest of my review to patch 16 still holds).

Paolo

> It's been dead weight for 8+ years and no one has complained about
> KSM+KVM performance (I'd also be curious to know how much performance was gained
> by shaving VM-Exits).  As KVM is the only user of .change_pte(), dropping it in
> KVM would mean the entire MMU notifier could also go away.
> 


  reply	other threads:[~2021-03-31 16:37 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 [this message]
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
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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