linux-kernel - Re: [PATCH v3 0/4] KVM: nVMX: Enlightened MSR Bitmap feature for Hyper-V on KVM

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Message-ID: <87r1bxmfw1.fsf@vitty.brq.redhat.com>
Date:   Wed, 03 Nov 2021 16:13:18 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
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 v3 0/4] KVM: nVMX: Enlightened MSR Bitmap feature for
 Hyper-V on KVM

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> Changes since v2:
> - Renamed 'msr_bitmap_changed' to 'msr_bitmap_force_recalc' [Paolo] and
>   expanded the comment near its definition explaining its limited 
>   usefulness [Sean].
>
> Original description:
>
> 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.
>

Ping?

-- 
Vitaly