linux-kernel - Re: [PATCH 10/18] KVM: Move x86's MMU notifier memslot walkers to generic code

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Message-ID: <d843bbd5-d3f0-3cda-aaa5-8e0fb1d69554@redhat.com>
Date:   Wed, 31 Mar 2021 18:36:44 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 10/18] KVM: Move x86's MMU notifier memslot walkers to
 generic code

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.
>