linux-kernel - Re: [PATCH 0/4] KVM: nVMX: Use vmcs01 ctrls shadow as basis for vmcs02

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Date:   Tue, 10 Aug 2021 19:39:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Zeng Guang <guang.zeng@...el.com>
Subject: Re: [PATCH 0/4] KVM: nVMX: Use vmcs01 ctrls shadow as basis for
 vmcs02

On 10/08/21 19:19, Sean Christopherson wrote:
> The goal of this series is to drop the vmx->secondary_exec_control cache
> without degrading nested VM-Enter performance.  The cache is effective,
> e.g. saves ~1000 cycles on nested VM-Enter, but confusing.  The worst of
> the confusion could be eliminated by returning the computed value from
> vmx_compute_secondary_exec_control() to make the calls to the "compute"
> helper more like the other controls.  But, the nested VM-Enter path would
> still have special handling for secondary exec controls, and ideally all
> controls would benefit from caching, though the benefits are marginal for
> other controls and thus difficult to justify.
> 
> Happily, vmcs01 already caches the calculated controls in the
> controls_shadow.  The only issue is that the controls_shadow may have
> dynamically toggled bits set.  However, that is not a fundamental problem,
> it's simply different than what is expected by the nested VM-Enter code
> and is easily remedied.
> 
> TL;DR: Get KVM's (L0's) desires for vmcs02 controls from vmcs01's
> controls_shadow instead of recalculating the desired controls on every
> nested VM-Enter, thus eliminating the need to have a dedicated cache for
> the secondary exec controls calulation.
> 
> Sean Christopherson (4):
>    KVM: VMX: Use current VMCS to query WAITPKG support for MSR emulation
>    KVM: nVMX: Pull KVM L0's desired controls directly from vmcs01
>    KVM: VMX: Drop caching of KVM's desired sec exec controls for vmcs01
>    KVM: VMX: Hide VMCS control calculators in vmx.c
> 
>   arch/x86/kvm/vmx/nested.c | 25 ++++++++++++--------
>   arch/x86/kvm/vmx/vmx.c    | 48 +++++++++++++++++++++++++++------------
>   arch/x86/kvm/vmx/vmx.h    | 35 +++++-----------------------
>   3 files changed, 56 insertions(+), 52 deletions(-)
> 

Queued, thanks (patch 1 for 5.14, the rest for 5.15).

Paolo