linux-kernel - Re: [PATCH v5 0/4] KVM: nVMX: Enlightened MSR Bitmap feature for Hyper-V on KVM

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Date:   Thu, 2 Dec 2021 13:20:47 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/4] KVM: nVMX: Enlightened MSR Bitmap feature for
 Hyper-V on KVM

On 11/29/21 10:47, Vitaly Kuznetsov wrote:
> Changes since "[PATCH v4 0/8] KVM: nVMX: Enlightened MSR Bitmap feature for
> Hyper-V on KVM (+ KVM: x86: MSR filtering and related fixes)":
> - Drop Sean's "KVM: x86: MSR filtering and related fixes" as they're
>   already queued, rebase to the latest kvm/queue.
> 
> Original description of the feature:
> 
> Updating MSR bitmap for L2 is not cheap and rearly needed. TLFS for Hyper-V
> offers 'Enlightened MSR Bitmap' feature which allows L1 hypervisor to
> inform L0 when it changes MSR bitmap, this eliminates the need to examine
> L1's MSR bitmap for L2 every time when 'real' MSR bitmap for L2 gets
> constructed.
> 
> When the feature is enabled for Win10+WSL2, it shaves off around 700 CPU
> cycles from a nested vmexit cost (tight cpuid loop test).
> 
> First patch of the series is unrelated to the newly implemented feature,
> it fixes a bug in Enlightened MSR Bitmap usage when KVM runs as a nested
> hypervisor on top of Hyper-V.
> 
> Vitaly Kuznetsov (4):
>    KVM: nVMX: Don't use Enlightened MSR Bitmap for L3
>    KVM: VMX: Introduce vmx_msr_bitmap_l01_changed() helper
>    KVM: nVMX: Track whether changes in L0 require MSR bitmap for L2 to be
>      rebuilt
>    KVM: nVMX: Implement Enlightened MSR Bitmap feature
> 
>   arch/x86/kvm/hyperv.c     |  2 ++
>   arch/x86/kvm/vmx/nested.c | 23 +++++++++++++++++++++-
>   arch/x86/kvm/vmx/vmx.c    | 41 ++++++++++++++++++++++++++-------------
>   arch/x86/kvm/vmx/vmx.h    |  9 +++++++++
>   4 files changed, 61 insertions(+), 14 deletions(-)
> 

Queued, thanks.

Paolo