linux-kernel - Re: [PATCH v2 00/10] KVM: Consolidate and optimize MMU notifiers

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Message-ID: <9376b453-be3a-f8b7-d53a-7e54c25161ce@redhat.com>
Date:   Fri, 2 Apr 2021 14:17:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>
Cc:     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 v2 00/10] KVM: Consolidate and optimize MMU notifiers

On 02/04/21 02:56, Sean Christopherson wrote:
> The end goal of this series is to optimize the MMU notifiers to take
> mmu_lock if and only if the notification is relevant to KVM, i.e. the hva
> range overlaps a memslot.   Large VMs (hundreds of vCPUs) are very
> sensitive to mmu_lock being taken for write at inopportune times, and
> such VMs also tend to be "static", e.g. backed by HugeTLB with minimal
> page shenanigans.  The vast majority of notifications for these VMs will
> be spurious (for KVM), and eliding mmu_lock for spurious notifications
> avoids an otherwise unacceptable disruption to the guest.
> 
> To get there without potentially degrading performance, e.g. due to
> multiple memslot lookups, especially on non-x86 where the use cases are
> largely unknown (from my perspective), first consolidate the MMU notifier
> logic by moving the hva->gfn lookups into common KVM.
> 
> Based on kvm/queue, commit 5f986f748438 ("KVM: x86: dump_vmcs should
> include the autoload/autostore MSR lists").
> 
> Well tested on Intel and AMD.  Compile tested for arm64, MIPS, PPC,
> PPC e500, and s390.  Absolutely needs to be tested for real on non-x86,
> I give it even odds that I introduced an off-by-one bug somewhere.
> 
> v2:
>   - Drop the patches that have already been pushed to kvm/queue.
>   - Drop two selftest changes that had snuck in via "git commit -a".
>   - Add a patch to assert that mmu_notifier_count is elevated when
>     .change_pte() runs. [Paolo]
>   - Split out moving KVM_MMU_(UN)LOCK() to __kvm_handle_hva_range() to a
>     separate patch.  Opted not to squash it with the introduction of the
>     common hva walkers (patch 02), as that prevented sharing code between
>     the old and new APIs. [Paolo]
>   - Tweak the comment in kvm_vm_destroy() above the smashing of the new
>     slots lock. [Paolo]
>   - Make mmu_notifier_slots_lock unconditional to avoid #ifdefs. [Paolo]
> 
> v1:
>   - https://lkml.kernel.org/r/20210326021957.1424875-1-seanjc@google.com
> 
> Sean Christopherson (10):
>    KVM: Assert that notifier count is elevated in .change_pte()
>    KVM: Move x86's MMU notifier memslot walkers to generic code
>    KVM: arm64: Convert to the gfn-based MMU notifier callbacks
>    KVM: MIPS/MMU: Convert to the gfn-based MMU notifier callbacks
>    KVM: PPC: Convert to the gfn-based MMU notifier callbacks
>    KVM: Kill off the old hva-based MMU notifier callbacks
>    KVM: Move MMU notifier's mmu_lock acquisition into common helper
>    KVM: Take mmu_lock when handling MMU notifier iff the hva hits a
>      memslot
>    KVM: Don't take mmu_lock for range invalidation unless necessary
>    KVM: x86/mmu: Allow yielding during MMU notifier unmap/zap, if
>      possible
> 
>   arch/arm64/kvm/mmu.c                   | 117 +++------
>   arch/mips/kvm/mmu.c                    |  97 ++------
>   arch/powerpc/include/asm/kvm_book3s.h  |  12 +-
>   arch/powerpc/include/asm/kvm_ppc.h     |   9 +-
>   arch/powerpc/kvm/book3s.c              |  18 +-
>   arch/powerpc/kvm/book3s.h              |  10 +-
>   arch/powerpc/kvm/book3s_64_mmu_hv.c    |  98 ++------
>   arch/powerpc/kvm/book3s_64_mmu_radix.c |  25 +-
>   arch/powerpc/kvm/book3s_hv.c           |  12 +-
>   arch/powerpc/kvm/book3s_pr.c           |  56 ++---
>   arch/powerpc/kvm/e500_mmu_host.c       |  27 +-
>   arch/x86/kvm/mmu/mmu.c                 | 127 ++++------
>   arch/x86/kvm/mmu/tdp_mmu.c             | 245 +++++++------------
>   arch/x86/kvm/mmu/tdp_mmu.h             |  14 +-
>   include/linux/kvm_host.h               |  22 +-
>   virt/kvm/kvm_main.c                    | 325 +++++++++++++++++++------
>   16 files changed, 552 insertions(+), 662 deletions(-)
> 

For MIPS, I am going to post a series that simplifies TLB flushing 
further.  I applied it, and rebased this one on top, to 
kvm/mmu-notifier-queue.

Architecture maintainers, please look at the branch and review/test/ack 
your parts.

Thanks!

Paolo